Throwing this out here to test the water.
We're soon going to be adding more variety to planet types, which will presumably include some more cold planets. Given that Unggoy are the primary workforce of the Covenant and they breed at insane rates in methane-based atmospheres, I see the potential for a methane atmosphere planet type (worded more elegantly in-game, of course) where after the Covenant research the taming of the Unggoy these planets have massive buffs to their population (total and growth rate).
This allows for more tactical decisions for their opponents as well as an econ buff for the Covenant itself. Does the UNSC risk a fleet in attacking this planet to reduce the fighting force of the Covenant? Does the Covenant station a fleet nearby to defend it and the buffs they gain from it?
SotP is always focused on adding more tactical depth to the game and I think this is an interesting idea to at least discuss...