1) Adam Warlock
Ever Since a shell looking like Adam Warlock's from the comics appeared in guardians of the galaxy, speculation has been rife that Warlock gets to appear in guardians of the galaxy 2, much to the the delight of fanboys. Further facts to fuel these rumours are that he has a major part to play in the infinity war arc from the comics.
2) Yondu Udonta
Yes you read it right folks! Yondu, the leader of the ravagers in the guardians of the galaxy. This has been hinted since the film released. In an interview director James Gunn admitted to the reporters saying "Yondu is not Dumb." In the comics he has been part of the team and has an affectionate relationship with Peter Quill aka Star Lord. Although he doesn't admit it but it has been hinted repeatedly. Firstly he does not deliver Quill to his father and then saves him from being eaten alive. Secondly he is easily convinced by Quill to help him stop Ronan. Also he doesn't seem surprised when he discovers that Quill has replaced the stone by a doll.
Director James Gunn has already admitted that there will be just one new member added to the team and the existing characters will be explored further. Judging by the evidence it can be clearly seen that YONDU has the best chance to join the team.