KI Jago could heal and it was a good example of it being done right (mira too), but they nailed almost every mechanic they touched imo. Part of that is because the core mechanics allow lots of ways to interact with that kind of bullshit.
In general though the problem with healing is it promotes really lame play and feels terrible. You can use it to always force your opponent to come to you, really shifting the balance, and if you fuck up it snowballs fast, to where suddenly the match is about making SURE you don't fuck up because they'll heal up and undo everything you've done.
All that said I do think it can be used. Mira really is a great example of the kind of character who should be allowed to heal. She could do all sorts of busted shit that was way outside the realm of fair, but it actually hurts her every-time she uses one of those options, to the point that you can literally have a mira beat the shit out of your for a round, poke her once, and win.
The "balance" was that the damage she dealt to herself could be healed (and only that damage), but the ways she'd get the heal would mean dropping either most of her damage or taking other risks.
Jago on the other hand just pops his xfactor mode and then all his projectiles heal on hit. It limits how he can heal both in timeframe, occurrence, and alters his combos/options so you're not just dealing with "random ass heal move"
To tie this back to mk and stop fanboying about KI, I think shang's healing just striaght needs a buff. There's enough conditions on it as is that I doubt it'd be too much of a mess if it didn't outright suck. The only concern is that since he's already a keepaway character you're messing with the "feelsbad" area again if you wind up with a strong zoner who, if you ever give them space, might undo all that work you did
