So how about a new unit type : SPIRIT
Units of this type would link to other standard units, and whenever one linked unit gains mana, the other unit would receive it as well… Their ability would buff the other unit in one way or the other, or do whatever you think they should 
I think with the implementation of units types and spell that target certain types, this would now be possible and very doable/viable.
Any thought?
Talking passive skills, are we?
If passive skills were ever implemented, there’s quite a few ways the game could be changed. Not good nor bad per se, but definitely change how the game is played.
Other possible passive skills:
Health regen - Recover (not gain) lost hearts each turn.
Mana balance - Any mana gained would be split with another team member.
Attack copying - Copy the attack of the opponent’s top troop.
Stat swapping - Randomly swap a random stat with a random troop… Randomly!