Perhaps it would be worthwhile to post my own thoughts about stage legality somewhere on the internet for others to consider. This thread seems like an appropriate place. As a disclaimer, I do not actively participate in the competitive community at all, but perhaps that means I can offer unique perspective.
From what I can tell, competitive play always develops very conservative stage lists, which I support. So I looked at SSBU's stage list to see what stages could possibly be legal, and despite having very conservative criteria, I was surprised how large my list ended up being regardless. Here are my thoughts on the potential legality of several stages, partly based on my interpretation of how hazardless forms will work:
- Battlefield: Starter for 1v1 and 2v2. Banned for 3v3 and 4v4 for small size. If "Battlefield" stage forms have identical terrain / blast zones / gameplay, they can be used as well, but any player can veto omega stages with especially distracting visuals (eg. Umbra Clock Tower).
- Big Battlefield: Starter for 4v4. 3v3 could maybe have it as a counterpick or starter. Definitely banned for 1v1 and 2v2 for large size.
- Final Destination: Counterpick for 1v1 and 2v2, starter or counterpick for 3v3, and starter for 4v4. The same rules for "Battlefield" stage forms would also apply to omega forms here.
- Kongo Jungle: Banned. Even if the hazardless version removes the barrel cannon, it likely won't make the main platform fully solid.
- Super Happy Tree: In the very unlikely event that the hazardless version completely removes the cloud platforms and makes the blast zones more reasonable (akin to the 1P version in the original game), it's a candidate for starter or counterpick for 2v2 and 3v3, as it contributes a unique but reasonable platform layout. But it'd probably still be banned for 1v1 for large size and 4v4 for small size. Banned otherwise.
- Dream Land: Banned for extreme similarity to Battlefield, and also for the weird terrain properties, if they were maintained when porting from SSB4.
- Rainbow Cruise: If the hazardless version stays on the ship and doesn't move or transition to anything else, this is actually a possible counterpick for 1v1 and 2v2. It contributes a unique stacked two platform layout and terrain asymmetry. However, the wall on the right side could easily be grounds for a ban, if it proves to be too problematic with Ultimate's mechanics, and it'd be banned for 3v3 and 4v4 for small size. Banned otherwise.
- Brinstar: Banned. Even if the hazardless version removes the terrain hurtboxes, it likely won't make the main platform fully solid.
- Yoshi's Story: Counterpick for 1v1 and 2v2. Hazardless version will be used if it removes Shy Guys but not Randall. Banned for 3v3 and 4v4 for small size.
- Fountain of Dreams: Starter or counterpick for 1v1 and 2v2. Don't use hazardless if it has still platforms. Banned for 3v3 and 4v4 for small size.
- Pokémon Stadium / 2: One of these (probably PS2) will be a starter in hazardless form (assuming completely static stage) for 1v1, 2v2, and possibly even 3v3. Banned for 4v4 for small size. Hazards versions are banned.
- WarioWare, Inc.: Starter or counterpick in hazardless form (assuming completely static stage) for 1v1 and 2v2. Banned for 3v3 and 4v4 for small size.
- Norfair: Banned. Even if the hazardless form removes the lava, it still has an unusually small main platform, and soft platforms with ledges.
- Frigate Orpheon: Banned. Even though the hazardless form removes the transformations, it has an extremely unusual layout with a large wall and very significant movement, and a semisoft platform with a ledge. (Shame that the hazardless form doesn't use the second transformation without the side platforms, as it would be perfectly legal.)
- Yoshi's Island: Starter or counterpick in hazardless form for 1v1 and 2v2. Banned for 3v3 and 4v4 for small size.
- Halberd: The hazardless version will certainly remove the Combo Cannon's hazards, but all else depends on whether or not the stage still transforms. If it remains on the Halberd's deck the entire time, it's easily a starter for 1v1, 2v2, and maybe 3v3, but banned for 4v4 for small size I think. If the stage still transforms, it's a lot more contentious. The walkoffs layout in the hangar is obviously problematic, though thankfully it is only seen briefly at the start of the match. The transformations are disruptive when they occur, and the smaller layout has a semisoft platform that enables sharking. Testing would need to be done to determine whether it is a counterpick for 1v1 and 2v2 or banned, and the small size of the flying transformation ensures it would be banned for 3v3 and 4v4.
- Lylat Cruise: Starter or counterpick in hazardless form for 1v1, 2v2, and possibly 3v3. Banned for 4v4 for small size.
- Castle Siege: Starter or counterpick in hazardless form (assuming completely static stage) for 1v1, and potentially a counterpick for 2v2 unless it proves to be too small. Banned for 3v3 and 4v4 for small size.
- Smashville: Starter for 1v1 and 2v2. Hazardless version preferred if it removes the balloon but does not freeze the platform. Banned for 3v3 and 4v4 for small size.
- Unova Pokémon League: Starter in hazardless form (assuming completely static stage) for 1v1 and 2v2, and possibly 3v3 as well. Banned for 4v4 for small size.
- Prism Tower: Banned for walkoff transformation and semisolid main platform.
- Arena Ferox: Banned for solid ceilings.
- Reset Bomb Forest: Banned. Even if hazardless form does not transform, it does not have a single, unbroken main platform.
- Mushroom Kingdom U: Starter in hazardless form (assuming completely static stage) for 3v3 and 4v4. Banned for 1v1 and 2v2 for large size.
- Kalos Pokémon League: Starter in hazardless form (assuming completely static stage) for 1v1, 2v2, and possibly 3v3. Banned for 4v4 for small size.
- Town and City: Starter or counterpick for 1v1, 2v2, and possibly 3v3. Hazards version preferred to keep the moving platforms. Banned for 4v4 for small size. Possibly bannable in general due to the platforms being able to drag fighters past blast lines.
- Duck Hunt: Banned. Even if the hazardless version removes the duck and dog mechanics, it should still be banned as it is in SSB4 for the unusual platform layout that promotes camping.
- Midgar: Starter or counterpick for 1v1 and 2v2, and starter for 3v3, in hazardless form (assuming completely static stage). Despite being yet another tri-plat layout, it does has actual unique merits (unlike Dream Land) due to its larger size. With more refined / whittled down stage lists, it could possibly have a place as the starter tri-plat for 2v2, instead of Battlefield.
- Dracula's Castle: Banned. Despite the hazardless version likely technically being unintrusive, its unorthodox and asymmetrical layout with two walls and a big ramp are undesirable. It also doesn't have enough unique merits to warrant inclusion, as there are several dual platform stages to choose from now.
Any stages not already mentioned would be banned for what I feel are very obvious reasons. Stage morphing is banned for being disruptive and causing lag. My rough assumptions for bans based on size could change if the game uses a reasonable stage scaling system for battles with different player counts, or if stage sizes have been significantly changed in their transition to this game.
Here's a sort of summary of what stages I think could possibly be considered for mainstream conservative stagelists. These are the "most lenient" possibilities for each stage, and the ones marked with question marks depend heavily on how the hazards toggle functions.
I really don't feel that any stages outside of this list are worth considering at all, unless there are some radical changes to how the hazardless toggle functions in the final game. Many stages here are labelled as debatably starter or counterpick, while very few are labelled as definitely counterpick, since I do support larger starter lists. Certainly, not all of these potential options would be included in final stage lists, and deliberation and testing would be needed to determine stage lists with balanced selections of stage and blast zone sizes and terrain layouts. (There's no way all three Pokémon dual platform stages would be necessary, for example, nor all the tri-plats.) But it is nonetheless encouraging to see 16 and 17 potential options for 1v1 and 2v2 respectively.
Sadly, 4v4 is hugely lacking in neutral stages that can accomodate this many players (and it certainly doesn't help that Pyrosphere, the best 8 player stage in SSB4, has been lost to Ridley), unless I'm overestimating how much space 4v4 matches need. On the other hand, there are many more stages here that might be big enough for 3v3, which gives a glimmer of hope that it could actually take off with this game somewhat.
The only stages in here that might need hazards on are Yoshi's Story, Fountain of Dreams, Smashville, and Town and City. The ease of simply keeping stage hazards off for everything kind of hinges on how it affects the former three stages, as I wouldn't lose sleep over Town and City having still platforms or being banned. Yoshi's Story and Fountain of Dreams are both just generic tri-plats if they don't have their platform movements, but if a refined stage list would not include them anyway, then there is no issue here. If Smashville's platform is frozen in the centre of the stage with hazards off, that might be more of an issue, as the moving platform is a relatively critical part of that stage's design. However, a single centre platform would provide a unique and solid stage layout, and I never did like how far Smashville's moving platform went out past the main platform's ledges.