With the current roster, there aren't a whole lot of characters remaining who could serve as viable commanders. You basically have Tosh, Niadra, and Selendis, but after that there aren't many left who could fit the bill. Using characters from the novels or comics wouldn't really work, as that'd be kinda awkward for people unfamiliar with those characters.
To that end, Blizzard is eventually going to have to expand the SC2 story past NCO with a new mini-campaign, allowing for new characters to be introduced and thus new potential commanders.
Either that or they'll have to discontinue making commanders.
