Many eons ago, I made a "faction diversity mod". One thing I diversified greatly was general faction themes. As an example, all Cybran units had lower vision range, but all units had radar (even Mantis) or radar better than what other factions had (such as on air scouts). Thus, Cybran stealth was toughest to deal with as Cybran. Aeon, oppositely, had no radar whatsoever, but all units had better vision range. I didnt like the suicide concept of the Mercy, so I changed it to act more like a flare, meaning it could linger near the front line for a couple minutes and provide almost soothsayer vision range.
I thought these ideas made matchups feel more different.