Since Romulus still exists in this universe, the one Star Trek apparently means to stay in for the foreseeable future - I do strongly think that unimportance ought to change.
Top of my list for fleshing out aliens from TOS, would come down to either the Andorians or the Romulans.
I can't say Andorians aren't more wide open for development, with just a few elements Enterprise established to blend in. But more than that, I'm keen for somebody to take the Romulans and do for them, what TNG & DS9 did for the Klingons, in everything since "Sins of the Father". Maybe even try to unseat the foul smelling barbarians as the Number #1 Star Trek villains.
It'll be a shame if they don't go that route, given a) the popular view built up since the late 80's of Klingons being more or less on our side, Federation allies off & on but mostly honourable and b) the Romulans having far less established about them.
Nero's actions - everything onwards from destroying the U.S.S. Kelvin, to returning and causing the Vulcan disaster - that would seem to be a perfect storm the Romulans wouldn't be able to resist taking advantage of. Fingers pointing at them after the first event in 2233, an attack they had nothing to do with, would likely have been enough to bring back out of hiding in a big way.
Blanks that could be filled in, if Star Trek had more than a couple of hours to play with...