It is a very cool idea, but there are not really any systems in the game currently supporting it (like a questing system you speak of). So there would be a lot to add. But I'll keep it in mind for adding progression
xD I did say up front that the ideas build on each other and could turn into a huge system. If you find the input helpful as opposed to annoying, just let me know where you'd want to focus brainstorming and what direction you want to go with it.
Again, I'm don't know how to code. Based on what you currently have in the game, I'm guessing the simplest way to do this would be to lock raiding buttons until a criteria is met.
To follow the above example, you currently have the temple raiding area. If there's a way to lock that and only make that selection available if you have a harem girl with a certain level of purity stat or after sending her to train at temple X number of times or something similar.
The rest of this post is under the assumption that locking a raiding area behind a criteria is doable.
Obviously this can't be done until after you've added in traits and stats in V3, then training methods in a later version, but I suspect at least having your outline of the direction you want to go in place will help with design choices you make along the way when choosing traits and training options to put in whether you go this direction or focus more on castle building or both.
So rather than a specific quest, it's just meeting criteria to unlock a raiding area. It could just be a higher level area instead of a specific boss fight. Then to simplify the idea of capping spell power behind rituals. Maybe your spell power is capped based on which raiding areas are unlocked. Alternately a boss fight raiding area could be unlocked based on the girl stats and beating that boss could unlock the new raiding area.
Cannon could explain it with idea of finding the scroll in the temple, while mechanics would just unlock a higher spell power ceiling for each area unlocked.
it ties together with the other post about making stats more useful than just numbers big.
I do think capping spell power rather than letting it scale infinitely will add more of an element of challenge and strategy to your combat system.
As it is the strat is to just pick a spell and level it to the point of 1 hitting anything while ignoring the others. Caps could force you to use strategy like the lust spell vs some targets and the self healing vs some, then raw damage against others.
Then having higher level caps gated behind something provides a sense of progression and purpose. Happy to help you look at numbers to balance where to cap for each tier of raiding area.
The same thing could go for castle building finding blueprints or doing the same thing with automatic blueprints coming available upon unlocking a raiding zone.
So building on your current system, building recipes that require stone unlock when you unlock the temple, recipes that require wood might be the ones you start the game with as the 1st / free raiding area would probably be the woods.