Carnophage

B for 2/2 that deals you 1 damage a turn. You can choose not to pay and get it tapped (although you wouldn't usually do this) so it's somewhat different from its Zendikar counterpart, Vampire Lacerator, although very similar. If you actually think about this 2 damage to opp - 1 damage to self situation, you will realise that it doesn't get you too far ahead in the reality of the game state unless you can push very aggressively so this really goes only into very aggressive decks.

Cataclysm

A very powerful lockdown card which would likely pop out in a GW beatdown archetype or something similar.  This card can be maximised by playing all 4 card types but even at a minimal of 1 land and 1 creature, the odds of the game being sealed at that point in time is very high, better if the opponent is tapped out. To make the card work better, cards such as Flagstones of Trokair or Yavimaya Elder that work when they enter the graveyard can be good. Floating mana like when using Upheaval can also be very deadly so playing it in a WB build for Tombstalker and even Soul Stair Expedition / (Oversold Cemetary) can be good too.
Greater Gargadon in red can also break the symmetrical effect by eating up the cards that are going into the graveyard anyway.

City of Traitors

Tapping for 2 colorless mana is definitely good but its drawback can be quite significant. It belongs to very specific archetypes that can make use of its "burst" mana such as a storm deck or perhaps an aggressive deck (say a blue or red aggro deck) that is not too color intensive (like WW)
On a side note, this card works well with the Ravnica block "karoo" lands but because of conflicting interests, you would probably not find them in the same deck. In any case, if you are going to play into the mid-late game, this is probably not the card for you.

Culling the Weak

A Dark Ritual + B that needs a sacrificial creature. On the flip side, this can also mean, your dying creature can turn into BBBB for you to use at a cost of B. A very possible card to play in a storm deck that has 0 mana creatures and cards such as Infernal Contract or Necropotence that can use the BBBB that comes from this spell.

Dominating Licid

Looking at this card, one can tell that it is a rather powerful creature as it can potentially remove 2 attackers from combat by blocking one and controlling the other. The problem would be where to put this card into since it's rather fragile as a 1/1. I suppose a UG deck would be the most probable house for this card, possibly as a sideboard card against a creature deck with little removals, seeing that the UG archetype won't lose too much board position if a creature is lost. I would directly compare this with Old man of the Sea and weigh the needs of the deck to choose which to play.

Ephemeron

This card looks dull in comparison with many of the other blue 6 mana fatties. However, being able to jump back to hand is a valuable ability. If you need an "undying" game ender, this is a card that fits into that description.

Forbid

Counter magic that buybacks. It can't get more irritating for people who hate counter magic. Combined with Ophidian or Shadowmage Infiltrator or simple card drawing, its buyback cost becomes almost negligible. Other cards such as Gush or (Meloku, the clouded mirror) also become more valuable with Forbid. Being able to maximize its use or having foresight into the possible next plays is an important skill when it comes to buyback cards. It is much more important here because you lose 2 valuable cards with the buyback cost. A simple play that could potentially be backbreaking could be a Remand on the spell being cast that targeted by a Forbid with buyback or just a counter magic targeting Forbid. Thus, knowing when not to buyback is the key to making this card work for you.

Hatred

This card does something very unfair. If you have more life than your opponent and you have an unblocked blocker, win the game if your opponent has nothing. Everytime we look at a card, we assume that the worst case scenario occurs and that clouds our eyes from the true power of cards and combos. Of course, I won't say this card is so powerful such that every deck that plays black should play this card but the converse Murphy's law of "everything that can go wrong will go wrong" should not apply all the time. Setting up your opponent by making him tapped out is going to be the key to making the card work for you. It's no doubt a risky card to play but benefits of playing it might be worth the risk. Just how much do you want to win immediately? I foresee this to be a great sideboard card as you never know who is more aggressive in game 1 unless your deck is made very very aggressive. I wouldn't mind playing this in a sideboard of a deck that has a couple of creatures that connect every now and then.

Keeper of the Dead

By stockpiling creatures in your graveyard, you will be able to Doom blade a creature every turn. The requirements are not difficult to create especially since black has cheap creatures or cycling creatures. The only foreseeable problem would probably which deck that this card can go into. I'm guessing that this belongs in a sideboard of a black heavy mid-ranged deck that runs a considerable amount of creatures or with a accessible constant graveyard removal. Some decks would just get decimated by free Doom blades every turn.

Keeper of the Flame

Just as how one can manipulate life total to set up a Pulse of the forge with pain lands, fetchlands and shocklands, Keeper of the Flame can also be used in this direction. The only problem is that she does nothing when you have more life than your opponent. Probably a card to help the monored deck (assumed from the RR casting cost) fight aggressive decks rather than against slow matchups.

Limited Resources

This card will annoy people who like to have lands out. It can potentially lock a player out from the 6cc and up range of spells. I think this is a card that belongs in a early-midrange deck's sideboard to suppress late game strategies.

Mind over Matter

The first card that comes to mind is Arcanis the omnipotent which would let you draw your entire deck with Mind over Matter.  Together with Winter Orb, it can lock out your opponent's mana while you get all your lands every turn.
With Lotus Vale out, Mind over Matter turns every card into a Dark Ritual.
Every card turns into a free Twiddle so it's up to you to use it.

I don't have many ideas regarding this card but I'm sure someone else might.

Null Brooch

2, discard your hand: Negate. If you have flashback, madness or have the ability to play "handless", this card would work wonders for you. The first two are self-explanatory but how does one play "handless"? Aggressive mid-ranged decks can use this card. Control decks with Sensei's Divining Top can make use of the artifact to play from the top of the deck.

Oath of Druids

This is a very powerful card which could support something like a lone Progenitus if built around. It is rather difficult to build a deck around this card unless the enchantment can be searched out with great consistency and can be almost 100% protected. As such, the most fitting colors would be a UGW (bant) color combination. This can be combined with Polymorph cards to increase the consistency of getting the Progenitus out. This is a "build around me" card which may or may not be worth all the effort. Only testing can tell.

Plaguebearer

A Gorilla Shaman that kills non-black creatures instead. Having the mana to kill the creatures you need is not easy but with this, you can clear tons of tokens and a bunch of low converted mana cost creatures very fast. I would think this as sideboard material against decks such as elves?

Price of Progress

This is probably the MOST painful burn spell that one can pick up, capable of dealing damage many times its casting cost. This will be very strong now since people still play a considerable amount of non-basic lands in their decks. Its power might drop when people start to move towards the basic / non-basic camp.

Ravenous Baboons

While we have an almost strictly superior Goblin Ruinblaster, this card has its own pros over it or Avalanche Riders. You can use Momentary Blink or reanimation spells with Ravenous Baboons to pop non-basic lands which would more likely be the target of most land destruction spells. Avalanche Riders is odd in the sense that you almost always want to pay its echo cost but that would slow you down, returning the lost turn from the lost land to your opponent. No additional costs on this guy to create such a situation. Not too aggressive but it does its job well.

Recurring Nightmare

Swap a creature in play with a creature in the graveyard. You can repeat this process for every 3 mana you have. With a card that spits out a few tokens, you could just use this card to reanimate your whole graveyard into play if you had the mana to do so.
For example: Swap a random creature for Siege-gang commander. Swap a goblin token for Siege-gang commander. Swap the other tokens for 2 other cards in the graveyard.
Cards like Yosei, the morning star would really mess the game up with a Recurring Nightmare. It can even be used as a simple reanimation card provided you have a creature out in play (which is not difficult at all).

Shattering Pulse

Shatter with buyback! I would think that this card might see some play if artifacts start to become more popular in the future. Not something the format needs at the moment but it's good to have.

Skyshroud Elite

A 2/3 for G most of the time since almost every deck plays non-basic lands. A very powerful addition to the card pool that is both a good attacker and a good blocker (since there are many 2/2s played in the format) but there are better blockers to block with. It's a green Kird Ape.

Skyshroud War Beast

This is a very powerful sideboard card against greedy decks that run heavily on non-basic lands. A big creature against a slow deck is always good. It's probably not maindeck material because of the possibility of it backfiring in the event that there are very few non-basic lands in play and not all decks run a ton of non-basic lands.

Sonic Burst

2 mana for 4 damage but you will need to discard a card at random. It's the same as casting 2 Shocks but you could potentially change that dead land into damage. The only problem is that you hardly want to cast it early and it might wreck your game when you do unless you have a fair amount of luck on your side. Still, the 4 damage can be rather valuable.

Spellshock

A very powerful card to use against blue mages or generally slower decks in aggressive red decks. Possibly a sideboard material that would see play as a Sulfuric Vortex #2 against slower decks. It works in a different way from Sulfuric Vortex which can be a good thing in itself. Putting Pyrostatic Pillar with this and Sulfuric Vortex into a deck could create a very suppressive environment.

Spike Weaver

Against a deck with little to no removals, this card buys 3 turns with its in-built fog effects. I can see it going into the sideboard of decks like the Heartbeat of Spring deck. Spikes have a cool interaction with Cauldron of Souls since the -1/-1 counter from persist cancels out one of the +1/+1 counters. Just saying.

Survival of the Fittest

The super creature tutor that will make its presence felt in the 100 card Singleton format. For one, like most good cards, a deck with it wouldn't need it out. With it in play, the game just goes to the next level. Traditionally abused with cards such as Squee, goblin nabob, Basking Rootwalla or Genesis to turn this creature swapping enchantment into a card advantage engine. Cards like Living Death or Reveillark greatly improve with this card. At the very least, you will definitely be able to find whatever creature you have in your deck that can best improve your board position with. Unfortunately, playing too few creatures with this card is not a good idea so this has to go into a deck with a healthy amount of creatures before it can work the way it is meant to.