I'm assuming this is mainly one-on-one team-ups right?
Rogue and Songbird: Because they have things in common and I want to see the banter between them.
Emma Frost and Moonstone: Same as Rogue and Songbird, kind of.
Jubilee and Jolt: Three for three. Hmmm sensing a theme here.
Kate Bishop and Finesse: It would be a lot of fun seeing these two work together and fight together.
Cassie/Stinger and Silk: I would really love to see Cassie team-up with one of the female spiders. Spider-Gwen could fit here too.
Scott Lang and a revived Eric Thunderstrike: Two everyday men fighting for the custody of their b****** children while always in the shadows of the legendary men who's mantle they took. Both also died.
Hulkling and Mar-Vell: Somehow, someway I'd love to see Hulkling team up with his father.
MODOK and Egghead: Two really weird science villains. I don't know if they've ever teamed up but I'd like it.
Yellowjacket and Black Ant: Two douchebag Ant-Man villains who really should try and form a 'sinister six' to get Ant-Man.
Captain Falcon and Patriot/Eli: A long overdue team-up that should have happened when Sam got the shield.
Magik and X-23: This one might have happened but during the periods when I was really deep in X-Men I wanted to see it. Laura had such a hard life before meeting Logan and Illyana grew up in a hell-like dimension. They'd have a lot to talk about as they fight together.
These are just the ones at the top of my head.