I think it'd be a bit of a downgrade if Lillie returned as just anything less than the Champion, honestly, in the event that these are sequels and/or build upon what SM have. I think there is a fairly strong argument for her actually being the main character of SM, and to have her appear in such a minor role after she was in 90% of the game after that, whilst not an unusual move, would be a downgrade. I didn't like her character and didn't think much of her development, but the point remains that an awful lot of focus was put into her, and it'd be pretty much wasted if that was all she amounted to, especially if her story is continued in USUM
I envision her having a mixed team with a blue/white kind of theme, though. Or maybe some Pokemon that mirror Lusamine's - Whimsicott to her Lilligant, Wigglytuff to her Clefable, Gyarados to her Milotic, Ursaring/Beartic to her Bewear, etc. It'd show a certain connection to her mother, yet symbolise her rebellion against her, too.