Aside from Dane Vogel the following I hope show up are.
Mr. Sunshine - Preferably referencing Montezuma from Robot Chicken. But that most likely won't happen. Revenge!.... Ha! Ha! Ha! Ha! Ha!
Dex - He died between Saints Row 2 & Saints Row: The Third in a game that was scrapped. Assuming he's dead despite that game being scrapped I want to see him in it.
Philip Loren - His death was too fast. I would've preferred something more memorable.
Shogo Akuji - Just so Gat could beat the crap out of him again.
Jyunichi - Almost the same reason as Shogo.
Matt from The Feed Dogs - He was too easily killed by a brick.
Josh Birk - He should be there just for being a bad actor.
Oleg Kirrlov - Despite being one of my favorite Third Street Saints he did have a shady past that wasn't delved into.
Tobias - Assuming he died from the earth being destroyed. Also Brandon Keener is same voice actor who did Garrus from Mass Effect.
Laura - Tobias's Wife.
Jessica - Okay I don't need any closure since the trunk was already closed....
The General - I found his death underwhelming.
Maero - Well he doesn't need to show up. Michael Dorn is still awesome though.
Kazuo Akuji - Actually I hope he isn't dead. His last words in Saints Row 2 were "When I escape The World will not be big enough for you to hide in!"
I don't have any listed from the first Saints Row because I've been too lazy to finish it....