For example, Johnny V's song "Shutaway" was a remake of an old song from 1994. I just remade a song from the mid 80's. One theme could be post a song you remade - the original and the remake.
Then there is always the "hey lets all do a cover of a song in the style of a different band" kinda thing.
I was in a group several years back that would challenge the members to certain recording projects and it was pretty fun You don't have to do any but it can be a bit of inspiration to do some recording, and its fun - gets you thinking outside of the box sometimes.

Yes we've done themes but not for a while. There was a 'Missing Person' theme a few years ago, and I remember a bunch of us posted tunes from our old band days or stuff we did decades ago. There was also a little challenge some time ago to write and record a new tune as quickly as possible. Those kind of things are fun.