I think Hopoo calls these stage features "permutations" - randomized props like open/closed gates, newt altar locations, or rubble blockages in pathways. All the variations of stages that fall short of being actual scene variants like Titanic Plains and Distant Roost have. Stage Variety doesn't refer to permutations by name because I don't think most people ascribe to that term. I do worry that using "variety" instead might lean too close to stage variants, which are not what Stage Variety adds. It adds 15 new stage permutations across the vanilla stages.
I made the first of these (the Aqueduct permutations) a long time ago as part of another mod also called Stage Variety. The original Stage Variety was focused on new stage "skins" and the new permutations were an afterthought. Skins would reskin the stage while also changing gameplay elements and monster and interactable pools. This might be sounding similar to a certain upcoming Gearbox feature..
A snowy skin for Titanic Plains that will never see the light of day
I dropped the stage skins so the permutations bounced around for a long time. For a while they were going to be part of Free Item Friday and that is where the bulk of the permutations were created. I realized that a Free Item Friday update was going to be a much larger challenge to tackle and so I pulled the now 13 permutations back into their own mod (which funnily enough is not StageVariety internally; I only realized that I could re-use that name right before release). I added the two Titanic Plains permutations for a cool 15.
The only good permutation in the mod! ft. Vibrant Visuals
The best permutations are the first 2 (Abandoned Aqueduct) and the last 2 (Titanic Plains). There is a lot of mediocre in-between, in my opinion, but I decided to ship everything because the whole mod is configurable anyway. I have very mixed feelings about the "sometimes removes a prop" permutations. The Aphelian Sanctuary platform will probably be especially controversial because 40 credits are taken away from the stage whenever the rock platform is disabled (similar to open/closed caves on other maps). These permutations exist because the RoR2 node system makes it difficult to "add" any terrain. But Nodes can be easily disabled at runtime, and with mods, because of the gate system. When I disable the Aphelian Sanctuary platform, I add all the nodes on the platform to a new node gate and also disable that gate, so that no interactables or monsters spawn floating in the air. Adding brand new nodes to a nodegraph through mods is likely possible, but absolutely not supported by Hopoo. I didn't add any new nodes with this mod which is why none of these permutations extend the vanilla stages - they are mostly replacing things or removing things.
Disabled nodes on Aphelian Sanctuary
Anyway long story short is that I don't plan on making many new permutations after these 15 because the design space is quite limited. I could make a system to inject new nodes at runtime but I feel that time would be better spent making totally new stages instead. These 15 permutations should be a fun way to spice up runs, though. You can see all 15 with pictures on the Stage Variety wiki. I am trying something new by putting the wiki on Github rather than Thunderstore. This lets me create the wiki prior to uploading the mod which means the information is available sooner. I worry it might make the wiki harder to locate if people aren't scanning the README closely, though.
If anyone out there is reading this, you are my strongest soldier. I hope to keep writing posts for new mod releases.