Characters I would actually miss:
01. Mordin (Most interesting by a country mile.)
02. Legion (More to explore. "Why use my armor." "?I don't know.")
03. Kasumi (Probably won't be back, but she's a damn classy lady.)
05 (tie). Tali, Garrus (While note great, they are regulars now, and I've grown accustomed to their faces, so to speak. Also, they are not actively irritating.)
Whiners extraordinare who should return, but not as squad members (they are tolerable in small doses):
06. Thane (Waaah, my sonnnnnnnn!!!!1!!!11!1!!)
07. Miranda (Waaah, my daddyyyy!!11!1!1!!)
08. Jack (Waaah, meeeeeeeeeeeee!1!!1!!one!!eleven!)
Characters who shouldn't return, but if they did, I wouldn't object:
09. Grunt (His self-discovery-through-killing story is finished; he should step aside for someone new.)
10. Morinth (Actually has potential for more storytelling, but she's probably dead in the games of most people, anyway. Realistically, devoting more time to her would be an inefficient use of Bioware's resources.)
Cardboard cutouts who deserve either a passing mention or nothing at all:
11. Jacob (Should play some role in a citadel advertisement for some kind of ab workout.)
12. Samara (Should be referenced, but not by name. Basically, references to the justicars will serve as proxy references to Samara. That'll do.)
13. Zaeed (Should die in a bland fire unmentioned and unmourned.)