Well first some sad news. Hazardless Spirit Train has a lot going for it, but it seems to have a camping nook:
https://www.youtube.com/watch?v=KjS7HBtecvA
See Dark Samus just stand on the front of the train? That wouldn't be too big of a problem from what we've seen of Dark Samus (frankly she looks bottom tier), but it's pretty likely characters who aren't bad will be able to do that too. Spirit Train was probably not going to be the easiest argument to win anyway, but it at least had a ton of potential so this makes me kinda sad. It is what it is though; I think we should still seriously look at the stage, but this takes its probability for me from "unsure" to "seems unlikely" in terms of whether it will deserve to be legal.
I'm going to say this about the concept of having legal stages in general. So the single most important thing to realize which is also important to getting better at Smash in general is that there is no such thing as a "neutral" stage. Every stage all of the time in every match-up is inherently helping one side and hurting the other unless it's a ditto I guess (Even then, Brawl with its 55-45 Snake dittos taught us many things...). In 4 a lot of people held Smashville on a pedestal, but then look at it. It was a remarkably good Sheik stage, probably her best. It was basically unplayable for Little Mac; the platform height was so thoroughly wrong for him that you needed Duck Hunt/Kongo Jungle in your legal stage list to have a stage worse for Mac legal at your tournament. Some characters control platforms very well and love Battlefield (as a Rosalina main, I knew a thing or two about that). Some did ladder combos and wanted every bit of lower ceiling they could get (Town and City in conservative tournaments, Halberd at events that had it). As with the Little Mac example, sometimes it went the complete opposite direction. Platforms weren't a positive at all for Sonic's mobility parameters; he hated Battlefield. Just remember that every stage is always skewing everything, and there is no "neutral" baseline.
What does this have to do with variety? Well think about the game's global balance. If you play on a very small stage list, then most likely your meta will settle down to one or two stages as basically the core of the meta. In 4 it was the two Animal Crossing stages, and the biases of those stages became the biases of the entire metagame. With diversity you average things out a lot more and the game's "true" balance can shine through. This is why you want big and small stages, stages with platforms and without, stages that move and ones that don't, etc.. You want to be as open minded as possible because different characters care about different things and a difference that may be meaningless to one character may make all the difference in the world to another. This not only is almost assuredly actually more balance than a restrictive stagelist can produce when you average it all out, but it maintains the integrity of events by assuring that the winning players win because they're better at the game as a whole and not because the rules were stacked in their favor.
Naturally there's a balance here too. Smash has many pretty poor stages. We have the obviously degenerate stages like Temple with simple optimal strategies that remove interesting gameplay. We have a lot of less obvious but still gameplay crushing situations like I was worrying about above for Spirit Train. While all stages are polarizing to some extent, not all stages are equally polarizing; Kongo Jungle in Smash 4 was a good example of a stage not really broken in any way but that was just very, very polarizing and almost assuredly made the game worse if you had it legal. In reality, the game "works" to at least some extent with anywhere from 1-60 legal stages probably. The best game is probably in the neighborhood of 30 based on our current knowledge of the mechanics of the stages in Smash Ultimate; the details are still breaking as we speak and will continue being uncovered for at least the first six months of the game's lifespan if not longer.
I also think we can learn a lot from our history if we care to actually look at the past. Here are a few important things to keep in mind:
-In the entire history of the community, we've never unbanned a stage. Even bans done for "temporary" reasons like the boat clip on Wuhu Island were not considered for legality again after the issue was patched. That being the case, it's VERY important to start liberal. Inevitably some stages from such a list will be banned with time, but it's just an unrealistic outcome to start conservative, discover you have too few stages, and then start adding. That's not how the community works, and if we put ourselves in that situation, we're probably just stuck for the game's entire lifespan playing a sub-optimal game.
-Objectively the starter-counterpick dichotomy does not work. I've said it until my face turned blue and then kept saying it, but I'll say it again. It's about how player strategy and rulesets ultimately intersect. Strategically you need to plan to only play on starters if you want to win sets 2-1 on starter stages instead of lose them 1-2 on counterpick stages. Since all the strong players understand this at least instinctively if not explicitly, starter stages are the only ones actually chosen in high level play. This causes play on counterpick stages over time to grow to be very sloppy due to poor stage knowledge which causes people to resent the presence of the stages which causes them to be banned. A counterpick stage is a "soon to be banned" stage; let's not waste everyone's time with those this time around please.
-The community does not really have a good track record of game design. We banned almost every stage in Brawl to allegedly nerf Meta Knight, and not only did his tournament performance not decline at all (it held quite steady), we created a monster in Ice Climbers who were suddenly #2 in the game but totally degenerate to play against with the handful of stages we had left. We basically argued and culturally legislated three whole characters out of the game in Smash 4 in the form of the Mii Fighters. The idea of us designing a narrow stage list that somehow is just the best possible assumes a collective competence on our parts that is frankly completely unrealistic, and it's not even that the Smash community is dumber than average so much as it's just a fact that large groups of people do not collectively perform quality design work. Our best hope is to avoid game design as much as possible and, wherever we can, defer to the wisdom of Sakurai and team in making the game the way it is for a reason and just accepting that wherever it leads us.
-Most of the community for Smash Ultimate is not currently playing competitive Smash. Most of them will be new players who will only begin to get into the scene after the game comes out; that has been what has happened with every game. In that sense, tradition is actually a lot less meaningful than you probably instinctively think; familiarity hardly matters when most of the real community won't be familiar with anything to start. We should focus on doing what's best for that reason; it's not just or even mostly about those of us here now.
-In the past a lot of the toxicity in the community has come from us fighting over the various problems games have given us and our inability to agree on how to play them. That toxicity has been a constant source of embarrassment for the community (we're pretty well detested in the broader FGC), and I can tell you from experience that it drives people out way more than it draws them in. Smash Ultimate for perhaps the first time in series history has really gone out of its way to make this easy. We have hazards off to give us variety and a certain tameness so everyone wins here. I think if you're a stage conservative, looking at the situation as "well this makes it even easier to have exactly what I want; let me force things that way as much as I can" is not a productive viewpoint toward growing the community. A viewpoint that will probably produce much better long term results is "now everyone can have the things they want most with all of us having to give up less than ever to get along". Let's take advantage of this golden opportunity generously given to us by the main dev team instead of squandering it by refusing to compromise with people who actually look at things differently at all.
