Well now that I've played the game, here's some of my own initial impressions on some of the more questionable stages.
Mute City: The main platforms are actually much bigger than I expected them to be! And the car platforms are around much more often than I thought. Their edges are kind of strange, but beyond that I find this stage quite playable as of now. -Optimistic
Tortimer's Island: A tad bigger than I expected. Could cause stalling shenanigans from faster characters. haven't seen how powerful the exploding fruit is yet. If it isn't too crazy, I can see this being a solid doubles stage if singles don't work out.
Reset Bomb Forest: First form seems fine. Second form has an odd layout, but otherwise doesn't seem terribly difficult to fight on. Walls that can create cave of life can be broken, which could make for some interesting strategy. -Needs more testing, but optimistic for now.
Tomodachi Life: Very unique stage that seems to favor vertical combat. Vertical projectiles will be quite useful since the foreground walls block the view. Easily legal unless circle camping becomes a problem, imo. -optimistic
Arena Ferox: Besides one particular transformation being a massive cave of life, this stage seems like a standard solid transforming stage. I see no reason to ban it as of now. -optimistic
Gaur Plain: Was sort of hoping the under platforms could help against walk-off camping, but initial impressions imply this isn't the case. Even if walk-offs aren't a problem on it, I feel the layout of the stage is just really freaking weird that there's probably something else busted about it. -Needs more testing, but very pessimistic as of now.
Rainbow Road: Very limited playtime here so far, but the one or two times I played it the Shy guys showed up like, once and were super telegraphed. Possible legal material? -Definitely worth testing imo, no opinion so far.
Haven't played enough Brinstar, Jungle Japes, or Distant Planet, but they may also be possibilities.
Again, these are initial impressions from less than a day of playtime. I'm pretty excited to see how this plays out though.