I'm pretty sure he wants Walt to go since Locke visited Walt. I'm pretty sure we won't see frank again. I'm sure we will see Desmond again, I'm not sure he would be part of the group that has to go back though. In a way i would almost be alright with not seeing Desmond again as part of the main story even though he is such a cool character just because i always wanted to see him make it, and he's in the best possible spot right now with penny
I think Frank is gone, as is Micheal. I think Jin is out there - could be a back-up boat or the other people from the island on the inflatable, whatever. And I think we will definitely see a lot more of Desmond. For starters, Ben is gunning for Penny after Widmore killed his daughter. There's bound to be a plotline with Penny and Ben and Desmond is pretty likely to be involved. Second, if the island requires Locke to be among those returning, I've got believe its going to require Desmond too.