Let's gather some ideas for the plot of the real final level. Feel free to write your idea. Note that the sublevels and hubs which were already made cannot be omitted.
Here's my one.
(Inspired by this
The level name will be "Abyss of Dichotomy", or alternatively "Trial of Dichotomy"(ASMT vibe)
General gimmick is that the structure seems to force players to choose one of two given options(two doors in a hub). But you can take an unexpected action and you will be in the state where you cannot access those two doors anymore. Then it will trigger the third door. Figuring out the exceptional, refusing choice can be an interesting small puzzle which can be an alternative for some super-hard rooms too. (like, the third door leads to an easier level.)
There are two functional reasons to come up with this. First, the difficulty of each room varies too much, and it's reported that some levels were excessively hard and unfair(some testers said they were never able to beat some rooms). Unfortunately the authors doesn't seem active anymore and there's rarely chance to have the levels updated to be easier. This method can flexibly accommodate these levels without sacrificing them.
Second, I'm expecting there will be 12-13 sublevels for the real final. If we just make hubs have two rooms, you have to pass through at least 6 tiers (+boss). Then the length of the level will be excessively long and exhausting. On the other hand, if some hubs can have more than two rooms, the number of steps can be reduced by 1 or 2.
To implement this idea, it's required to measure the difficulty of each room. And I'm sure it's also required for other possible plots.
Here's my rough measurement which is a bit personal too. 1 is the easiest and 4 is the hardest.
B3: being updated, 3 or 4? (previous 2)
BD and BE will be completed sooner or later (said by authors)
e: You are welcome to make your levels easier, if you want of course!
e2: made the description more clear
1: obviously easy
2: hard for the first, but becomes pretty easy after learning
up until now the levels were able to beaten by testers (almost)
Higher than 2 means the levels weren't beaten by some testers.
3: also doable after learning, but possibly harder than 2
(2~3: the levels which deserved 3 previously, but became easier afterward. They require re-testing)
4: still hard despite learning