I think one of the problems we're getting at is how i feel like its far more challenging to create rewards for mono than it is to create a cost for allies.
If we look at rules like obsec, that benefits hordes far more than it does elite armies. The mono reward would need to be fashioned on a codex to codex basis. Im not against that but then your balancing whole extra set of things.
I can’t remember if which thread I’ve said it in now but I think there’s a wide range of strategic buffs you could give to an army composed of a single codex if it was facing an allied force. The idea being, don’t try and compete with allies at their own game, create a counterbalance so that mono lists had a big advantage elsewhere. Things like bonuses to first turn rolls, or deployment maps or allowing first turn reserves or choosing who goes first in deployment are just a few areas that might be worth considering.