honestly, i think you're overthinking it too much.

most people who are interested in slice-of-life modern fantasy things aren't really looking for a whole lot more than cool relationship dynamics and fun threads. they're not really wanting to deal with a lot of the micromanagement that comes from site shops or leveling, which means putting them in is either going to attract a demographic i don't really think you want (aka the people who do enjoy gameifying their rp experience), become an annoyance for your community (such as if you locked certain high profile character types behind a shop purchase), or end up neglected entirely because it's largely a moot point (like if you could purchase a forever fc reserve but no one really needs it or wants it).

if you really desperately feel the need to add something, hold regular thread roulette events (so people are sort of forced out of threading in their own little bubbles with the same partners) or just the occasional light event in general for a brief change of pace or add in some cute little collectable mini-sprites for things like "you've threaded with x character type!" or "you're in a relationship! cute!" because honestly a lot of people tend to really like stuff like that and they're neat little collectables that can be rewards without being a pain for newbies to get and not detrimental to anyone who genuinely doesn't care about them.

like. real talk, your members joined because what they saw on the site interested them and they liked what was there. adding in too much new stuff to that core formula is going to negate some of that appeal so i would just... let it be what it is and call it a day.