With the initial six and Karax, Stukov, and Matt Horner in consideration; who else would we like to see, and what would their specializations be?
I'd like to see acknowledgement of SC1 some more, so my top pick for representation from that game would be DuGalle. The UED quite clearly established their combat preferences throughout Brood War, and it'd be easy to adapt them to Co-op. DuGalle can get Marines, Medics, Goliaths, Siege Tanks, Wraiths, Valkyries, and Battlecruisers. His gameplay style would focus on aerial dominance with Valkyries, Wraiths and Battlecruisers, with his ground units serving as support. His upgrades could give the Medic Restore and Optical Flare back, Goliaths their upgrades from the WoL campaign, Siege Tanks could get an anti-air mode or alt attack, Wraiths can get their WoL abilities, Valkyries can have speed boosts and a damage buff (perhaps armor-piercing attacks), and Battlecruisers can get back their WoL abilities like defensive matrix and missile barrage.
EDIT - I've done a lot of commanders in this thread, here's a list.
Admiral DuGalle
Dehaka
The Overmind
Ulrezaj
Duran
Aldaris
General Duke
Mira Han
Arcturus Mengsk
Matt Horner