The only requirement is for the cards to have a normal, modern card frame so they don't necessarily have to be Modern legal but most are. I also prefer interactive games over broken/combo style decks so there aren't a ton of fast mana producers or two card combos although there are some listed below. I also do not like cards that can be cast without mana so the cube will probably never contain cards like Force of Negation.

These are the general archetypes I am going for although some are more clear and linear than others.

Colors	Archetype	Sample Cards
	Control
	Tempo
	Aggro
	Ramp
	Midrange
	Taxes
	Spells
	Reanimate
	Tokens
	Value

There are probably a bunch I'm missing but these are pretty good.

Description	Cards
There is no Splinter Twin but you can assemble Kiki-Jiki, Mirror Breaker plus either Zealous Conscripts, Restoration Angel, or Pestermite to create infinite haste tokens by repeatedly untapping it with the creature being copied
Melira, Sylvok Outcast or Anafenza, Kin-Tree Spirit plus a creature with Persist is really good. If you can also get a way to sacrifice creatures, you can loop them a bunch or even infinitely
Skullclamp and repeatable 1-toughness creatures to draw a ton of cards
Fetchlands like Misty Rainforest can search for any Shock, Tri, or Surveil Land like Temple Garden, Thundering Falls, or Indatha Triome that shares a color since they have basic land types.
Ancient Greenwarden or Crucible of Worlds can continually play lands from the graveyard that have a sacrifice ability. Fetchlands like Scalding Tarn or Escape Tunnel can be played and sacrificed every turn to find more lands. Wasteland can be used to destroy a non-basic land every turn.
Mimic Vat and creatures that have good enter the battlefield abilities you can use every turn.

Tibalt is awesome so every card is included in addition to the normal 540.