Casting Demiplane allows the caster to have:
A Medium-only door ('Must be this size or smaller to ride - NO OGRES ALLOWED') that vanishes when spell ends / after one hour.
Creates a permanent (30' × 30') room of stone &/or wood walls.
And that it can be reconnected to if known or using a tuning fork.
However, my problem is that the created demiplane is too small and barren. I want more space for activities. It might be possible with mordenkainen's magnificent mansion, but I'm not sure if it even works, and the duration is limited (24 hours) without some way to affect the passage of time in the demiplane.
So, is there any way to add or expand —ideally permanently— to the space-size of this 30' × 30' demiplane?
Answers may include: any of the sourcebooks (PHB, Volo's, Mordenkainen's, etc.), tweets from Lord Crawford, Sage Advice, any 5e Unearthed Arcana... or whatever one can find that is RAW, quasi-legal or even slightly more tested than outright home-brew.