A cube for all the cards and archetypes that I really wanted to play, but always ended up in the "maybe pile" when constructing decks.

Anticipated archetypes:
WU Affinity
WB Midrange Lifegain/Drain
UB Mill
UR Spellslinger
BR Madness
BG Aristocrats
RG "Big" creatures
RW Equipment
GW Go-Wide/Tokens
GU Self-Mill

Three-colour macro archetypes:
WUR Artifacts
RUG Goodstuff
BWG Aristocrats
BUG Graveyard Value
BWR Midrange Grind

(With a splash of incidental Zombie and Goblin tribal.)