A thought I had, which after messing around with and properly setting up-works, I had this idea about bringing the leveling system to normal units, but not in the sense of making them heroes. Just normal units that gain experience in a battle, and if succeed or survive, gain from it. Having increased damage, hp, etc. So for example, if you have a single Archon thats kicking some zerg arse, it gets rewarded for all the kills and surviving it, having increased damage, shields, and hp.

Granted this idea could be OP, because if a powerful army killed another, and still lives in its fullest, then it just became that much more powerful.

Thoughts?

P.S I was thinking of adding this also to defenses, such as photon cannons, bunkers, etc. (units inside the bunker would level).

It really depends on the map's combat system how this will turn out.

If you throw this system into a default melee game, you will buff protoss because their units tend to live longer than for example Zerg units.
But it will encourage stronger, beefy units like carriers and battle cruisers because they can benefit the most with these systems as the bonus is concentrated into one unit and kept alive until the whole unit dies instead of having the bonus split on multiple marines that crumble during the fight.

In general, it shouldn't have a to big impact in a melee game except if you design all combat dynamics around that system.
It will benefit players that keep their units alive.

But sadly that's hardest for Zerg players as most of their units die like flies during combat while other races tend to keep their units alive more easily.
So this bonus shouldn't be inceased on a kill basis, it should use an xp amount, so units need to kill multiple Zerglings to become stronger. Else Zerg becomes much weaker than the other races.

In some situations these changes can have a big impact, too.
For example in PvZ, a Zealot that kills a Zergling early game might start to 2 hit zerglings after leveling. Basically this would make the Zealot stronger by 33% versus Zerglings.

I was thinking of applying it to melee games.

As for protoss I would agree. Since Protoss is stronger, could make it so protoss units that are killed give more xp from another race equivelent. So something like..

EXP GAIN

Zerglings = 1xp
Marines = 2xp
Zealots = 3xp

I think a better idea for these is to not give the leveling system to cannon fodder units. So marines/marauders, lings/roaches, zealots/stalkers. Instead give them to higher tiered units, such as siege tanks/battleships/thors/ghosts, Hydralisks/Ultras/Broods/corruptors, Archons/Collossus/Carriers/mothership. Something like that for the units who actually level. The cannon fodder units it may not be worth giving it to them, just because they are that, cheap cannon fodder, they wouldn't be hard to kill unless mass, and massing them may make this idea OP.

Or if all units have leveling abilities, just require a lot of xp to level. Higher the tier unit, the more xp.

Of course this can drastically change gameplay, and could create different styles of playing. Such as a small group of ghosts sniping out units to gain xp/levels. Or a collossus becoming considerably more powerful.

However you can change what units have what attributes, so you can have it so some units only become harder to kill. Such as a ultra gaining more armor & health, siege gaining more HP, with slight splash dmg increase. Collossus with just more damage. Not all units need to have the same stat gains.

That reminds me of Command and Conquer's veterancy system. I quite enjoyed that, although I don't think it would mesh well with SC2 proleague it'd be fun in more casual games and campaigns.

Oh I remember that. Fun times

I already made it into a mod and played it with some friends, they liked it.