Well, first I should say congrats on finishing this game. I'm definitely having a blast with it, especially with how you've managed to combine a lot of elements that aren't usually found together in most platformers.
But as the topic title says, I think it would be even better if you can re-fight old bosses. I'm only about 22% through the game, and I've only beaten 2 of the bosses (Too busy collecting everything else, really), so I don't know if you let us do this later on in the game. But to me, it just feels weird that we're able to replay everything in the game, each level, each mission, and yet we're locked out of boss fights after we beat them.
I'm not particularly certain what others feel, but I had a blast fighting the bosses. They're just as fun as everything else in the game, so I'd really like to have fun fighting them again. So, if it really is impossible to re-fight them, could you possibly add it in so you can re-fight them? If you can and I just haven't gotten far enough in-game, feel free to disregard this message.