10 Outrageous Guardians Of The Galaxy Vol. 2 Theories

Drax's motivations for opposing Ronan are based on the fact that he killed his wife and daughter, which was later to be revealed on the instruction of Thanos. However, there is a possibility that the Mad Titan didn't actually kill his daughter, and instead raised her as his own.

Thanos has multiple adopted daughters, so there is precedent for it already, but the key motivation behind the theory is that that is actually the case in the comics. She goes by the name Moondragon there, and although she's a human (Gunn has said other Earthlings won't be appearing), they've changed Drax's origins in the movie, so could well do the same for her. 

With Thanos likely to provide some opposition for the Guardians in the next film, even just in the background, there would be some sense in having her appear, and as an extremely powerful fighter and telepath, she'd be a very interesting character to introduce, either working against or for (or both) the Guardians.