Definitely Horner/Valerian for fleet based, as Bio/Mech/Ghost were covered (Raynor/Swann/Nova). Zerg will definitely be Dehaka, now that Stukov is confirmed. And for Protoss we'll surely get Talandar in all his Purifier glory.
I'd put my money on Talandar as we've already got a Terran commander: Nova, and i know that they dont necessarily release them in a race order to keep the same number of commanders, but surely they have enough purifier assets to work with a purifier based commander.
About there not being any units left for old zerg commanders like the overmind i'm sure they can keep coming up with abilites and reskins for the same units, one of the things i was thinking about a while back was the possibility of the Overmind using basically Zerg "Merc" units, like the devouring one, hunter killers and even the yggdrassil, but who really knows.
Also from the few responses that i've heard none seem to reference the fact that those characters died in the first place.}
I for one just hope that at some point there are enough assets from all commanders to build up sc1 entirely with blizzard models, even if some still want to use their old crappy models...
Yeah there are plenty of unused/half finished units in the editor. Now that the infested bunker unit is confirmed it is definitely a possibility! They could use the shredder, tentacle monster, the Zerg artillery, creep tower, he'll storm missile turret, replicator and many more! As they are doing with Stukov they might even be creating new units!
Another thing that would be cool, but unlikely is a commander that commands hybrid (the only thing I can think of that would do this is Naurd but he is amon's follower. Also a Xel'Naga commander, this may be oros (I know the spelling of his name is wrong), or mayor be assended Kerrigan!
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Its been a while since any commanders have been released since stukov so Im really hoping for another very soon and would be amazing if it was Dehaka and him be a playable commander...we've not seen any primal zerg yet and I think this would be awesome. I haven't played co op in a little while because of staleness especially when you are above like lvl 60-70 mastery and all commanders are level 15 and no new commanders or maps in a lgood while...they were on a good role and keeping us in suspense with news, teasers, and hints for new commanders every month or 2 since alarak and now it seems they just stopped...they have sooo many possibilities like the list made by the OP but also consider Selendis and others as well