The previous question alluded to here: “Watch rowlet lose its flying type upon evolving and become a grass/fighting type or something even weirder. That would certainly be interesting”
See, I don’t think that sort of thing is likely, purely because the starters are the one place where, more than any other (except maybe the mascot legendaries), they want mass appeal, and while that doesn’t exactly rule out weird and quirky designs for the evolved forms, I believe it makes Game Freak more likely to incline towards safe choices. Looking back at past starters… they really just don’t tend to “do” major reversals in aesthetics and design as they evolve. I suspect there’s kind of an expectation that, when you choose your starter, you should pretty much know what you’re getting. It’s supposed to be your partner, so if it evolves into something radically different and you don’t like it anymore, the design has kinda failed, in that particular respect.