^ Basically this. It'd be an extremely odd move for Game Freak to omit a third game or even a sequel to Sun and Moon given how many open avenues that they've seemingly (whether it's intentional or not, idk) left for a third game and/or sequel to cover. For example, Lillie can return as an actual full-fledged Trainer and perhaps you can have some sort of battle with her! I think that'd be really cool.
I can't really speculate what her Kanto journey would be like, other than what's already stated by Wicke (I think) in that the very reason she decided to go to Kanto in the first place is not only to become a Pokemon Trainer but to also visit Bill to see whether or not he can get the UB's poison out of her mother's system, so there's that.