I really have a lot of feels about them. A lot of feels that are hard to make words. But I just think... I love Rapunzel's ability to help people grow, and she'd be able to do that so wonderfully with the Once-ler. I think she'd be really sweet to him and patient about working through all of his problems, but also totally not afraid to tease him or bruise his ego sometimes, which I think would be great for him, tbh. And he'd be good for her in that he's very much a dose of the real world, of how things don't always go how you plan, BUT she also sees in him the ability to pick up and move on. I just think they're dynamic of her being excited or teasing and him just kinda bringing her back down to reality is adorable. I think they'd inspire each other creatively, which is great, and be able to have silly conversations about stupid things that don't really matter, but feel like they do to them. And just. Good things. Things that make me smile.
So, for canon!Billy/canon!Rapunzel, I love the idea of this happening post-Dr. Horrible Canon. (Because Rapunzel and outlaws is a canon thing. But not really because of that.) I don't know, I love the idea of Rapunzel helping him get over his guilt, but not....Over Penny? I legit feel like she'd respect that she'd always hold a place in Billy's heart, but she just has SO MUCH good inside her to show him and help him find hope again and a different way of making the world less of a mess. And Rapunzel would just love all of his inventions and potential and how smart and handy and awkward he is. Plus he would sing with her; major bonus to her. They'd probably be a pretty schmoopzy pair for a while, with less, if any, of Rapunzel's teasing, because I think that's what Billy needs post-canon and she'd recognize that. But then, eventually, if Billy seemed he could take it, they could have a nice little domestic relationship where they kind of poke fun at each other and point out bad ideas, but always support each other and know the other cares about them.