I've been trying to come up with something to represent the cyclic nature of the Nile, a concept that seems appropriate to Ankh-Theb. The 'thallid' mechanic does a good job of it, but with other effects that don't just involve making saprolings. The mechanic would be used in Green and Blue, the colors that would be associated with the river culture. To differentiate the colors, Green gets growth counters only on itself, and has better effects on removing counters. Blue can put growth counters on any permanent you control, but has weaker effects on its cards.
Pest-Ridden Crop G
Creature - Plant
At the beginning of your upkeep, put a growth counter on ~.
Remove three growth counters from ~: put a 1/1 green insect creature token onto the battlefield.
1/1
Sobek Farmer 1G
Creature - Crocodile Druid
At the beginning of your upkeep, put a growth counter on ~.
Remove three growth counters from ~: Add GGG to your mana pool.
1/3
Field Irrigator 1U
Creature - Human
At the beginning of your upkeep, put a growth counter on target permanent you control.
Remove a growth counter from ~: Target creature gains islandwalk until end of turn.
1/1