At first I thought of this idea for just tier 1 units but it could be applied to any unit. Basically I'm thinking of types of passive abilities based on a unit's position relative to surrounding terrain, surrounding units, and or it's own status. So for example: A zergling gains +1 attack or an attack speed buff when more than 10 other zerglings are within a certain range of it. A zealot gains an attack speed buff when it is in contact with a terrain obstacle such as a wall. A marine's attack speed decreases when being hit by a melee unit but its attack is buffed when it isn't being attacked by a melee unit. I've also been thinking about this sort of ability with protoss unit shields. If all protoss units are buffed a bit, but when their shields are depleted, they are debuffed in some way. I created these examples for the sole purpose of demonstrating my concept and am in no way proposing their implementation into the game. They might be op. They might be superfluous. They might be nerfs. They serve as simple illustrations.
These passive abilities can really help define unit roles as well as personalities in accordance with the campaign. A great implementation of this type of ability is creep and its speed boost to zerg units. It fits very well with the zerg "swarm" style and adds strategic depth to the game. A zealot fighting harder when cornered/ in a bad situation fits pretty well with their character. Zerg units being stronger in numbers like a swarm also fits although that might encourage deathball styles too much.
These unit interactions can lead to cool micro/ positioning tricks comparable to drop micro or reducing unit surface area with terrain to minimize zergling damage and are somewhat understandable by casual viewers like Blizzard wants without being too moba-y(tell me if you think it is :/). For the zealot ability concept, microing zealots to hug terrain obstacles to increase their cost efficiency would be pretty cool. This could also help with issues in balancing specific units while still keeping their distinct role in the game.
I just formulated this idea when I was idling around in the shower or whatnot and I'm not a great theorycrafter or even a player at that. Also, I don't really play the campaign so I'm not too familiar with unit personalities; I just thought of some stuff that I already knew from curosry lore reading. This is just a product of my brainstorming. Maybe these changes are too drastic and are better saved for StarCraft 3(let's hope lol). Feel free to comment your questions, thoughts, and or suggestions.
Some more brainstormed abilities:
+ Show Spoiler +
-reapers behave differently(not necessarily better) alone than in groups, the result of them being "covert" scouting style units
-stalkers debuffed when attacked with melee units, "I'm here in the shadows" type thing
-queens get buff when next to other zerg units