Bowsette - Ok, hear me out on this... Bowser + Super Crown + Super Bell

The redhead tan version is just what some people think would be more aesthetically appropiate for a monster girl version of Bowser, even though the Super Crown actually doesn't turn anyone into a "cute girl" version of themselves but makes them look exactly like Peach, otherwise Peachette would have pink hair to match Toadette's pink braids and all (don't think I'm saying people who like the redhead version are wrong, it's just an observation, I don't really care about these kind of details).

As for why we don't see Bowsette combined with other power-ups… well, I say Bowsette's design is a big part of her appeal, so it makes sense that most artists don't want to mess with that too much or it just wouldn't be the same.