When the announcement initially came that there would be 68-72 characters in Avengers:Infinity War, many were wondering where would they pull those extra heroes from. Because Marvel not having the rights to any of the X-men characters amongst others could make that number kind of hard to reach.
Promotional art for the film was posted online and it featured the characters Yondu played by Michael Rooker and The Ancient One played by Tilda Swinton. So we see they can have characters to pull from.

But In an interview with CNET, co-director Joe Russo explained that Infinity War will feature a lot of unexpected characters. Here’s what he had to say:
“There’s going to be a lot of unexpected characters. There are more surprises packed into this film than any of the others. Part of that is character interactions, new interesting characters, characters that have made small appearances that are really getting fleshed out in this film. A lot of people enjoy the experience, so I don’t want to give anything away, but certainly you can expect lots of surprises.”
To date, the MCU stretches across almost ten years with a roster of seventeen films made so far, including the upcoming Thor: Ragnarok. Also let’s not forget that There are connected tv shows on network television and Netflix (Agents of S.H.I.E.L.D,Jessica Jones, Luke Cage, Daredevil, Iron Fist, The Punisher) There have been many characters to be featured in all of those films so the possibility of who could fill in the ensemble is immense.