Captain Marvel and... the Silver Surfer?
http://metro.co.uk/2018/03/20/has-the-silver-surfer-just-been-confirmed-for-avengers-infinity-war-alongside-brie-larsons-captain-marvel-7401061/
There has been a lot of speculation over this since the beginning, but it looks like Joe Russo has pretty much now confirmed that Infinity War will feature Captain Marvel making her debut in the MCU. Although this should probably be taken with a grain of salt, it seems to have merit (it comes from the director’s own mouth). And if it does turn out to be true, I know some may consider this unnecessary in a movie already packed with Marvel character after Marvel character. But Infinity War being Captain Marvel’s introduction into the MCU - similarly to how Spider-Man & Black Panther made their debut in Civil War before they starred in their own movies - makes sense. And yes, I can see it work.
*update*
Now I’m reading that her role in IW has been cut and the interview with Joe Russo is meaningless since it’s a year old. Oh hell!
The more far-fetched appearance? The Silver Surfer. That would be insane! Maybe nothing but a rumor but this rumoring can be fun.