I think Exorcism being able to hit Ogre Mages could be interesting.
Exorcism nullifying OM mana, or unlusting units would be awesome
Except that wouldn't be exorcism anymore we might as well rename it dispel at this point.
I mean one could argue it may be fun to have skeletons as the primary spell for death knights, a strong shield instead of a heal for paladins or haste and why not summon demons for axe throwers instead of trolls regeneration and divine intervention for mages so that they can revive units. Soon enough that would not be anything like war2 anymore we can achieve balance without changing the mechanics nor the way the game works.
I created that post because I saw a lot of people talking about balancing things but really they mostly suggest we change the game. Warcraft 2 is a game that first is not badly designed, second is not that badly balanced either, and third which remaining success relies primarily on nostalgia.
People come back to the game that they remember and loved, almost every old player remembers paladins have heal and exorcism it is iconic they love these spells they remember what you could use it for, most people remember heal as being good because if felt good throughout the campaign, most of them didn't ever use lust as efficiently as we do.
Almost none of them remembers exactly how much heal was able to heal and how much mana it cost, they certainly don't remember how powerful exorcism was and how much mana it cost. Players tend to have a memory bias they remember the game as being more balanced than it really is but they do remember very well what spells are and what they were used for.
People want to play the game they remember they come back for these cool mechanics, if it happens to be more balanced that is very good because either they wont remember or they will welcome the change because it will still be war2 but if it feels different that is where you start to loose people big time.
That is why here I am talking balance changes, and numbers tweaking, not mechanics changes or re-design (except for heal not having cap to behave like exorcism because that people wont remember and as mentioned before it should always have been the case).