what I kind of think might work well are those "try different styles" challenges (on deviantart were a few back in the days but idk if they're still online) like one is chibi, one manga, one is the simpsons, or adventure book covers, stuff like that (or you pick favourite creators and try to draw in their style)
the reson why I say this even though it is adapting another style is because even though we draw in different styles, something that is uniquely us will sneak into the drawing. they might look unrelated at first glance and yes, artists can have multiple styles, but there's always little things that are a continuety.