I think that this is a very good idea to implement as it would give more incentive to bring in money for the server, which is more motivation to keep it running with consistent updates and new content. Obviously donating should never feel like a requirement though, so nothing should scream "p2w" even though its a significant achievement or amount donated. Basically improving the qol for high donators. I would also be very please to see a custom title option brought back even if it comes with a restriction like that, I was upset to see it leave in the first place. As for the donation shop at the location, here are a few of my ideas:
https://runescape.wiki/w/Balloon_goat
https://runescape.wiki/w/Balloon_sheep
https://runescape.wiki/w/Balloon_cat
https://runescape.wiki/w/Balloon_dog
https://runescape.wiki/w/Golden_Gnome
https://runescape.wiki/w/Golden_gnome_hat
these are just examples and would just be ways to display your dedication/loyalty to the server.