Like you said, public progression is low and that's why creativity is low. There aren't a lot of actually competent developers in the rsps realm because there are not a lot of modern resources on how to create competent code. I think a custom server (but not dreamscape level custom) with some nice custom features could really breathe some life in to the general. There are quite a few ideas in my head about how to bring some pep in to OSRS' step, and I have slowly been making progress (on a 634 pvp server though lmao) that I think would set an example of how to fill a sorely needed niche in the pvp server area of things.
I think one of the biggest things lacking in RSPS in general is balance between systems and unique ideas. These days when I think about unique it's usually something very unsubstantial and lacking the sort of mental impact that good content needs. RSPS has the potential for a lot of variety while also keeping the variety balanced with stuff near it's intended power level, while OSRS' base is much much larger and has to be much more rigid for the level of player retention it has gained over the years. There are many RSPS and there are only a few experimenting and it hurts to see because variety is the name of the game. I think it's perfectly okay to draw inspiration from OSRS but you need to add your own flavor to it or it will become indistinguishable from all of the other OSRS emulation servers. More developers need to study the concept of horizontal progression vs vertical progression and try to create a game with a good hybrid. The diablo series of games are actually an extremely good example of a well thought out progression system and I've been wanting to see something even remotely similar put in to the community.