Who are some characters that you wish could have joined your squad? It could be Wrex or Liara reluctantly refusing, or someone who died unexpectedly like Okeer or Jenkins. It may even be someone unrelated to the squad like an enemy you wish you could have talked down.
Personally, I wish that Wilson could have been allowed to be given a second chance. He is a fairly powerful tech and biotic, and could even of had a "Rivalry" with Jacob or Thane(Jacob seemed willing to help him, however, so thane would be a good one).
Another I wish would have joined is Okeer. I don't understand why he just had to die. It would be awesome to have him on Tuchanka and all of a sudden the hostile Krogan distrust you even more for bringing back a mad scientist, who must have done some pretty bad stuff to be kicked off Tuchanka.
I don't have anymore, other than it would be cool to get a Mech, either a YMIR or LOKI. It would be cool, you choose a mech, and get three to support you along with one YMIR. They can't be revived, but they are restored at the ending of each mission. Upgrades give them other weapons and what not. And only the YMIR would have abilities and different weapons to choose from in a mission.
Anyway, those are what I wish I could of had, what are yours?