Very interesting thread. I'll posit a couple of observations, based on my own experience, if I may.
First, if the 'joy' of music has dwindled, I wouldn't necessarily try to fight it (whether by collabs or anything else). This happened to me after I left working in the studio. For about 5 years I didn't listen to any music, and most tragically during this time I also took my collection of irreplaceable music awards to the local tip. How I would love to have those on my wall now. I only stumbled back into writing when I got my ancient D-10 down from the loft one afternoon when my son was learning to play. I'm eternally grateful for the guys on this forum (especially Reece) for the assistance when I did.
My point here is that as 'artists' we can't always turn the vibe on or off so no problem for me if I check out for a while. Of course, if you are working in a professional capacity you're kind of committed or burn out (which is what happened in my case). Unplug the equipment and do something else - some gaming, kayaking, socialising, painting miniatures (Warhammer 40k are always fun) - whatever floats your boat. Come back when the ivories sing again.
Second - if the music vibe is still there, but there's a writing block do something different but related. I recently got much more interested in mastering so spent some time exploring those techniques with some great results. I read a bunch of books on music theory and harmony. I went back and remixed some of my material to focus on making them much more dynamic (pans, automation etc). And I picked a couple of VST synths to dive into and learn how to program.
Works for me. I tend to have periods of intense creative bursts for a few weeks, followed by a fallow time. Fallow is good.