I agree, they might need some sort of a condition to keep them in check. Basic lands have already been done, maybe permanent colors this time? In that case the abilities can be a bit stronger as well even without that many ways in the environment that can exile them from the 'yard.
Maybe something like this based on Desire:
Creature - Elemental Incarnation (Rare)
At the beginning of your upkeep, if Desire is in your graveyard or on the battlefield and you control another black permanent, lose 1 life and draw a card. So long as people want from others, Desire will push them to do so.