Seeing as though the ROM hack has had a sudden leap in development, these are some other ideas.
Beta Archive-inspired things. I know the channel is dead but the story is unfinished. What would the earliest build of Super Mario 64, specifically before the Summer '95 build.
Youtube channel. We could all have a youtube channel for the rom hack, posting footage of the hack and updates. Of course, this doesn't have to happen straight away as much more development has to happen.
A real life story inside the game. Ok so hear me out here. The player is playing the game as normal, until there developer note in one of the signs. There are more signs throughout the game as a conversation between two developers and friends. The game soon tears their friendship apart and one of them becomes angry and frustrated after the developer is put under pressure from the game. The final sign before the last Bowser battle reveals a real twist to the story. This was sort of inspired by SM64BA's Wing Cap Tower GUI ending with 'But it's all your fault'.
Hidden Floor: Behind one of the level-less paintings, there could be a door to a fourth-floor-esque area named the Hidden Floor. No idea what could be there but it could be an early third floor.
Music. Not really a plan but a question to Quiot Riot, the composer of the hack. Will the music use a SM64 Soundfont?
Also, these are the levels in each floor
Koopa's Battle 1
Courtyard (Need 3 Stars to get in)
Secret Slide (First Floor Slide but Betafied)
Basement (Need Koopa's Battle 1 key)
Deep Jungle (New Level)
The Fortress (based on scrapped Castle level)
Koopa's Battle 2
Second Basement (Need Koopa's Battle 2 key)
Cavern Land (HMC)
Winter Tower (Chroma Tundra)
Second Floor (the key to the second floor is in Winter Tower which serves as a boss battle).
Waterfall Land (TTM)
Pirate Ship Land (New level)
Clockwork Tower (TTC)
Koopa's Battle 3
Hidden Floor (Hidden behind a level-less painting on Second Floor)
Eternal Fort (same name)
The Rabbit Hole (Mips' Hole)
Plus, I can't decide on the name of beta Bob-omb Battlefield. Which one: