There are many possible plots of the next Guardians of the Galaxy movie,and let me share some of them.
1. Iron Man's appearance
The original comics shows that Iron Man joined Guardians of the Galaxy and fought with them, but I think that it could be happening in the next Iron Man movie(if released)or in the [Guardians of the Galaxy 2](movie:1081113)
2. Ronan the Accuser's appearance
Ronan died at the end of the Guardians of the Galaxy first film ,but he may not be dead and there is a significant chance that he will appear in the next Guardians of the Galaxy film.
3. Moondragon's appearance
She is the is the daughter of Drax who is thought dead, but she had an important place to play in the comics.
4. Mantis' appearance
5. Phyla-Vell's appearance
If you find more ,comment to let me know.