I saw someone mention Roll as an Echo Fighter for Mega Man, and I'd like to talk about it, as it's as interesting a proposition as it is awkward. Roll has been a clone of Mega Man in the past, but Super Smash Bros. very obviously sticks to the source material a heck of a lot more than Marvel Vs. Capcom 2 does, with Mega Man being one of the culminations of that. All of Roll's other playable appearances do not give her a buster, and instead have her use her iconic broom or...whatever she got in Mega Man 11.
The exceptions are a Japanese only mobile (and I mean like, pre-smartphone mobile) port of Mega Man where she did have a buster but couldn't use Robot Master weapons for whatever reason, and Mega Man X DiVE where every character seems to have various different arm canon graphics even if they already have an arm canon. It's also worth noting that even in Marvel Vs. Capcom 2, she didn't have an actual arm canon, but rather, put on an attachment whenever she wanted to use Roll Buster or related moves (something similar happens in Mega Man X DiVE where her arm canon disappears into the ether during animations that don't use it, which implies that it is also an external attachment). Even in Mega Man X DiVE though, she does still have her...uh...whatever she got in Mega Man 11, so the cleaning device that I can only assume is supposed to be a broom is still treated as Roll's "thing" (she is still a helper robot after all).
So with all that in mind, would it make sense for Roll to be an Echo Fighter of Mega Man at present? I honestly don't know. If she did, I'd say replacing the Mega Buster with Roll Swing is a must, and I think it would make the most sense if you stuck with Marvel Vs. Capcom 2's logic of her using an external arm cannon attachment for attacks that require it. The rest is kind of a question of whether or not that would work, and how much of that can be avoided through changing the attacks themselves.
You do also have to keep in mind that a canon playable Roll is kind of an inevitability at this point (with this being something both the fanbase and the devs are interested in pursuing IIRC), and could change how she should operate in Super Smash Bros. as well.