Just to lay out possibilities floating on the surface, we could easily get Hybrid faction f.e. Each race can be divided into 3+ (some rather countless) factions, for example:
Terran - Raynors' Raider (main) into Dominion/UED/Han's marauders/etc
Protoss - Aiur (main) - Tal'darim/Nerazim/Purifiers
I can't even list how many things can be done with Zerg, from per-faction mutation, focus on specific units/no base or one HQ research type to entire faction such as Stukov with Terran/Zerg hybrid armies.
Anyone remember C&C series, Generals ZH/Kane's Wrath? So, SC2 can do it WAY better.
We already have some sort of that in Co-op, which is fun at some extent, but bringing this stuff in player vs. player environment would be better IMO. Lots of units and tech are campaign exclusive and countless other stuff is just unused. Most curious that assets already exist including anims, voice overs, etc. It just needs few models to be added (while Blizz have no problem releasing pretty new skins every so often) and a little polish.
Other point that is kinda sad to me that melee customs are dead, plain and simple, while there are SO MANY awesome maps just sitting there useless. Anyone who wants to play SC2 in versus mode has to endure several maps chosen for the ladder over and over and over again. I perfectly understand that it's best that way for balance and competitive play purposes, but so much content can't be just wasted...
I can write whole essay on how to design it and bring more life into it, which would be pointless, since I'm pretty sure Blizz knows and understands all these possibilities better than any of us, the question is however: what are the real reasons they not doing anything about it? Since it doesn't require over the top resource investment for widening attractiveness of the game for more casual/average crowd uninterested in ladder seasons only. Plus since game went F2P, I'm pretty sure lots of people would be willing to pay for each faction any reasonable price. They would be killing like 10 birds with one stone with that move. General idea is that since this won't have main competitive focus, perfect balance wouldn't be such a priority as 3 main races for ladder.
Also, I'm curious, how community feels about it, would any of you guys be interested in something like this or it's just my personal pipedream and I'm full-time nutjob?...