Well so far our most likely candidates are:
Captain America
Iron Man
Thor
Hulk
Hawkeye
Black Widow
Quicksilver
Scarlet Witch
Vision
Falcon
Iron Patriot
Winter Soldier
Drax
Gamora
Star-Lord
Rocket
Groot
Ant-Man
Captain Marvel
Black Panther
Dr. Strange
Spider-Man
Less likely but also possible:
Inhumans (Royal Family)- However with the film coming out after IW2 it looks less likely now
The Defenders (Daredevil, Iron Fist, Jessica Jones, Luke Cage)- However these are predominantly street level heroes so their inclusion in a galactic spanning event are not as likely
Depending upon inclusion:
Wasp
Loki will play the Mephisto role at Thanos' side, only to betray him.
Nebula will also likely be at Thanos' side, only to betray him.