Alright, so while the issue of prowess is still up in the air, I thought I'd throw out some ideas for blue. Maro talks about how blue is lacking in a creature combat mechanic in common, so I want to feature a new mechanic to aid in that goal, but I don't want it to be the only thing blue is about in this set. The blue faction is partially represented by bards, and partially by rogues/assassins, so I thought of two things to give both sides an identity.
The only actual keyword I'm planning on having is not named yet, but I'm thinking enervation or sleepstrike or something to that effect. Basically, when the creature with the ability deals combat damage to another creature, that creature is tapped and doesn't untap on its controller's next untap step. Basically, it's another form of evasion in blue, but hopefully put on cards that are defensive in nature so it doesn't seem aggressive. I like it though because you can keep an attacker tapped to sneak your own guy through next turn, so it feels tricky, and very blue.
Another mechanic in this vein I was experimenting with was called aetherstrike, which also triggered on combat damage to a creature, but instead of tapping them, it returned them to their owner's hand. Now I'm not sure if this is more or less powerful than the other. On one hand, it seems like it is because it sets the opponent back on mana and keeps that creature from swinging for a turn, but on the other hand, they get to cast another creature (in case of any abilities that would care) and possibly set off ETB triggers with it. This is the "drawback" they've been implementing in some sets recently, with cards like Roaring Primadox. I don't know how often this would really be a problem, but it might seem less useful overall because of cases where you don't really want to bounce your opponent's creatures, but you would like to block. You guys can let me know what you think about either version.
Okay, now for the mechanic that will define the bards' side of blue, it won't have a name because it's unnecessary and I'm already going to have 5 different keywords I believe. What it does is this: the creature (it will only appear on creatures) can tap for a cost to activate an effect and you can choose not to untap it to continue its ability. They've used this type of mechanic before on many creatures throughout Magic's history, and I thought it would make a lot of flavor sense for bards to be singing a magic song to do whatever and as long as they keep singing, the magic keeps working.
An example of this: