Jungle Japes water is way strong this time around, at least in Brawl with swimming you lost all momentum the instant you left the water, but in Smash 4 the water's ridiculous momentum stays even if you jump out of it. I suppose it's not dissimilar to having an extremely high lower blast zone, but it does make recovering a series challenge for characters who have a mostly vertical recovery.
If that klaptrap is actually truly random this time around, though, this stage is a definite instant ban.
My personal opinion on what stages are worthy of being legal:
Battlefield
Final Destination
Arena Ferox
Yoshi's Island
Reset Bomb Forest
Prism Tower
Brinstar (probably a counterpick)
Tomodachi Life (good luck circle-camping this with all the drop-through floors. Unique stage design lets you approach from anywhere. Seems to actually be helpful for slower characters like Ganondorf.)
Unova Pokémon League (maybe a counterpick, though. Reshiram's Fusion Flare can really shut down some characters, such as Ganondorf, but otherwise the stage seems pretty reasonable.)
Spirit Train (occasional walk-off is a thing, but mostly the stage seems pretty legit. Only worried about certain small characters being able to camp on the front of the train, at least until a metal train comes along and forces them to leave or risk being drilled to death. Lack of a lower blast line on the front of the train might be a bit of an issue as well. Maybe just a counterpick?)
Distant Planet (Counterpick. Main platform interacts oddly with Greninja's Shadow Sneak, sometimes stopping it in place regardless of how far the shadow moved. The springy main platform also makes it more difficult to space attacks properly.)
Tortimer's Island (Probably a counterpick. Place is huge and the slopes prevent projectiles from crossing. Also Villager's Neutral Special and items. And Rosalina's Down Special and items.)
Mute City (Lacks a lower blastline, though the damaging floor hazard KOs off the top at around standard kill percent. Undervalues Meteor Smashes overall, while possibly overcentralizing aerial combat. Possibly a counter-pick, but pretty solid overall. Little Mac can navigate this stage effectively, but he has to wait for the platforms to line up in his favour, as he doesn't really want to do any jumping that doesn't involve hitting an opponent already on the platform above with Rising Uppercut.)
And the list of "maybes":
Rainbow Road (Are there specific telegraphs for the cars when they come in from the sides instead of the back? I'm having trouble anticipating how high they're going to take the jumps, and I haven't been able to find any safe zones other than the air well above the platforms.)
Jungle Japes (Are the klaptraps random or not? If they're not random, it's counterpick material.)
Gaur Plains (Fighting is centralized near the upper and horizontal blast lines, also walk-offs. Lower platforms are easy for characters with reach to defend against characters who lack reach. Lower blast line is very far down compared to the platforms, especially the upper platforms where most of the fighting takes place, making it potentially very easy to recover from Meteor Smash attempts.)
Magicant (Flying Men seem really pretty easy to neutralize, as they flinch from weak attacks for much longer than a fighter does. Needs some testing in a competitive setting. Also, platform along the bottom, though unlike CPUs you're unlikely to be saved by it when it's offscreen to the left or right. Platforms behave like the clouds in Brawl Skyworld in that you can Meteor Smash people right through them. Overall analysis: Overvalues Meteor Smashes, potentially overcentralizing stage hazard, occasional platform along the bottom to save characters. Important question: Is the platform along the bottom truly random or not?)
Green Hill Zone (walk-offs are definitely not an issue on this stage, the blast lines are super far offscreen. The half-pipe shape is weird though, and the damageable floor interferes with moves like Raptor Boost.)
Gerudo Valley (The lower blast line basically rises up and eats you as the bridge is repairing, which can be quite frustrating. Not random at all though, so most likely not an issue for most of the cast. If it is legal, it's pretty definitely a counterpick because of exactly that, along with the walkoffs and the damageable bridge encouraging play near the walkoffs while also interfering with attacks like Raptor Boost.)