The block mechanics encourage playing multiple spells in the same turn, and so the mechanics tend to enable this with cantrip-like effects and other ways to allow a player to play multiple spells a turn without depleting their hand.
1) Crescendo (Keyword on spells) - When you cast this spell, if a spell with lower converted mana cost has been cast this turn, Draw a card.
Examples: 2R Instant - Deal 3 damage to target creature or player. Crescendo. 1GG 3/3 Creature - Crescendo
Notes about Crescendo: - Plays well with spells from every other magic set, isn't only relevant in this block - Note that Crescendo's condition can be met even if the opponent plays the lower cost spell, so it creates tension for the opponent on what spells he/she should play on a given turn. - Crescendo is designed to challenge the player's normal routine (especially in limited) where they try to "curve out". Sure a 1GG 3/3 is fine on turn 3, but is it better to wait until turn 5 and cast it after a 2-drop? - Creates interesting deckbuilding incentives for limited. Often most 1-drops are too weak for limited play, but given enough Crescendo cards, you might be eager to get any barely-decent 1-drop in your deck to enable Crescendo.
2) Bloodsong (creature keyword) - Whenever *this* deals combat damage to a player, you may reveal the top X cards of your library and put a non-land card with CMC equal to or lower than X into your hand, where X is the amount of damage dealt. Put the rest on the bottom in any order.
Notes about Bloodsong: - Plays well with spells from every other magic set, isn't only relevant in this block - The amount of cards revealed and what cards you can select might need some balancing. The core of this mechanic is "When this creature connects, you get a chance at getting an extra card in hand". Variations include having X be the creature's CMC, or only being able to find cards with CMC equal to X, or strictly lower than X.
3) Harmony (threshold mechanic - static ability)- If 3 or more permanents you control share the same non-zero converted mana cost...
Examples: 1G 2/2 - Harmony - If 3 or more permanents you control share the same non-zero converted mana cost, *this* gets +2/+2. 2U Instant - Draw a card. Harmony - If 3 or more permanents you control share the same non-zero converted mana cost, Draw 2 cards instead.
Notes about Harmony: - While the wording might seem a bit verbose, the fact that this is a threshold mechanic makes me think it will pretty easily "grokkable". In the same way you check to see if your opponent has 3 artifacts in play for Metalcraft or 7 cards in the GY for Threshold, you would check to see if your opponent has "Harmony". - The non-zero part is necessary so that it can't be triggered by lands or tokens. Could take out the non-zero part if you want tokens to be able to enable Harmony, but would probably have to weaken the benefits of Harmony if that were allowed.
4) Convoke (Returning mechanic) - In a block that encourages you to play multiple spells each turn, there needs to be a mechanic that enables playing those spells by providing more mana. What an excellent time for convoke to return! After dropping small creatures to enable Crescendo and keep your hand and supply of spells flowing, they can be used to Convoke even larger spells.
This is all I have time for right now. I'm going to try to spend some more time figuring out how to work this Wiki thing to get links where they need to be and connect to the appropriate "hubs" (no clue...). Any help accomplishing that would be much appreciated :)
Please feel free to comment!