I have an argument that Mission could still be alive regardless of Revan's alignment.
With an LS Revan, she lived to the end.
With a DS Revan though, she was "killed" on the whatchamacallit planet, right? Maybe not. I don't have any numbers available, but I'd assume a good proportion of people who played DS ordered Zaalbar to kill Mission. If that could be made "canon" for the DS Revan, then Mission could have survived. Sure, Zaalbar obeyed the order somewhat, but in defiance he didn't land a killing blow. Revan, with everything else going on and Carth still alive, didn't notice this. Hey, she was busy with trying to take over the galaxy! Or if she did notice, she could have just decided to leave Mission laying there to bleed to death and besides, there was nothing that waif could have done at that point to thwart her plans (edit: I didn't think of this part before, and I like it much better than Revan not knowing - it's darker to let Mission bleed to death and it would show Revan feeling unstoppable by that point. I also neglected who saved Missions life, and that could be either natives or stranded Republicans from a downed ship).
Fast foward to KOTOR 3 - Your PC is searching for Revan, and has been given directions to go meet someone on a certain planet who may be able to help.
If Revan was LS, you meet the Mission we all know and love, but older, more mature, more experienced, and a definite asset to your party who wants to help find her friend. I'd also like to think she became a Jedi, but that isn't important.
If Revan was DS, you meet a very different Mission. A dark and scarred version who has revenge on her mind, and so has been training and planning all these long years with one goal in mind - killing a specific Sith Lord. And depending on what alignment you're playing, you could cultivate her darkness or help her find peace and redemption.
So yes, I'd like to see Mission come back if there is a KOTOR3 too, and as a very useful and interesting party member.