Setting Ridley aside for a moment...
I'm in the "Metroid Other M is an abomination" camp, and I'd like to see references to that game removed from Smash Bros., except for maybe a few random trophies and the original Nemesis Ridley music track. (I can take or leave the other two Other M music tracks.) Basically, I really hope that Samus's design gets changed to what it is in Samus Returns or to whatever it will be in Prime 4. I also hope that the Pyrosphere stage gets cut in favor of an SR388 stage (from Samus Returns), or again, an area from Prime 4. But, we'll just have to wait and see how things go.
As far as Metroid stages, I really wish they would get away from making nearly all of them have acid and lava themes. The only stage that bucked this trend was Frigate Orpheon, which was cut in Smash 4. Even Pyrosphere has somewhat of a "lava" theme going on, and Brinstar Depths from Melee had Kraid standing in something like acid when he originally rose from a bed of spikes in Super Metroid. And while it is true that Samus does have to deal with quite a few high heat / lava areas, as well as a few areas that contain acid / poison-like substances throughout the Metroid series... There are still so many more interesting locations or environments Samus has had to traverse that could potentially be used as a stage. (For example, just off the top of my head: Phendrana Drifts, Torvus Bog, Phazon Mines, any Space Pirate bases or research labs [Tourian perhaps], Planet Phaaze, Maridia, etc...) To be clear, I don't have a problem with Brinstar or Norfair. All I am saying is that
Metroid stages need more variety. But yeah, hopefully Pyrosphere gets cut in favor of a non-lava/acid area from Prime 4 or Planet SR388, but we'll see.
Also, when it comes to music, I have been and still am surprised that the "Lower Norfair" track from Super Metroid has not been remixed or included in Smash Bros...
This track was remixed for Magmoor Caverns in Metroid Prime...
...And for the high heat / lava areas in Samus Returns.
If this track does end up being included in Smash Bros this time, I hope it is either ripped directly from the Samus Returns remix directly above (it seemed really intense within the context of the game at the time I was playing it), or given similar treatment for a new remix. The Magmoor Caverns remix is not bad but seems a bit subdued for a fighting game.
And finally... Ridley... There's a lot I could say about Ridley being playable in Smash Bros, but the Internet has collectively beaten him to death more than Samus has, so I don't see a point in participating in an echo chamber. It also doesn't help that we keep getting leaks / rumors from fairly or semi-credible sources that he is a playable character, one of which is very recent at the time of this post. These leaks and/or rumors are all very good and exciting and all, but it could be setting a lot of people up for disappointment. Only time will tell. Personally, my expectations are tempered.
So, all I will say when it comes to Ridley is that if the Pyrosphere stage does get cut, I would think it would help Ridley's "chances" of being a playable character to an extent
*** (and also get rid of an Other M reference and another "lava" stage as a bonus.)
I think it helps Ridley's "chances" as it would be kind of strange to have Ridley as both a playable character and a stage hazard.
***...But then again, Donkey Kong is both a hazard in 75m and a playable character, and another Villager will occasionally appear in the background of either Smashville or Town and Country. ...However, the DK from the arcade version and the DK from the Country series are not technically canonically the same, just like Other M Ridley and the Ridley from, say, Super Metroid aren't canonically the same either. So while I want Ridley to be playable and Pyrosphere to be cut in favor of a newer Metroid stage, it's very hard to say if these two things affect one another. There's just no way to know what Sakurai and/or the development team are thinking until we know more information about the game, whether that's at E3 in a little under a month or whenever the game is released this year. Again, only time will tell, but here's to hoping for the best!