I am of the belief that restraints breed creativity - those restraints can always be later lifted after enough cards are created and the set is more-so in the development phase than the design phase. So here are the restrictions I am working with.
1) All the cards must be creatures.
2) All the cards must have unique creature types.
3) All cards must have legal creature types as of the release of Guilds of Ravnica.
4) All cards must be mechanically unique (except for evergreen keywords such as flying, haste, trample, etc.)
Primary anticipated problem - Instants are an important aspect of the game
Anticipated Solution - Creature mechanics that can double as instants (Morph, Exploit, Channel, Cycling, Bloodrush, etc.) however, there are only so many of these sorts of mechanics.
The Set is currently 2/3 done, 100 cards, but I think its already time for input, so go ahead, slaughter my self-esteem.