More legit room expansion (with both population limit and how many people we can have in bed) would be really nice. The prequel's small room upgrade made in story sense with it being a city manor with limited space. But in this story we are supposed to be land owners, a much bigger pop limit would give us way more space (pun intended) to collect a waifu of each race and maybe some special ones with good trait.
Speaking of which... the current room upgrade system using higher tier material makes sense form a game design perspective, but not really an in story reason. Why can we only make more rooms with magical materials to increase pop limit when people has been building more living spaces with common wood, stones, or even mud through out history?
Pretty sure you don't want to fully gut the current room upgrade system for something new. But maybe something to consider if there'll be a Strive 3. Maybe allow us to build more rooms with basic materials, but using higher grade material gives bonus similar to the current crafting system. Or maybe having room upgrade (that gives bonus) with higher grade materials.