Running on fumes, so just going to rehash an old whim of an idea: open-world housing.

Each zone has a few different 'hide-aways' that open into player housing zones, each themed differently. Think 'building up Revenant's Toll / Idyllshire, but player-based. Add in bridges, etc. The main idea being that works under the guise of creative housing while actually prototyping server-wide dynamic world construction and events for 4.0 or 5.0, which then utterly reorient the way you think about FATEs and such.