It reminds me of one of the best things we ever did in Loudness War rehearsal, which was Not My Job Day. I forget whose idea it was or why we did it, but for that day's rehearsal no one was allowed to play their usual instrument or sing their usual leads. The room had keyboard, banjo, ukulele, violin, sitar, harmonica, melodica, and mandolin in addition to the standard drums/bass/rhythm/lead lineup.
We slogged through our set, knowing it would sound weird and loose and letting that be okay. We laughed a lot, because what else can you do if you've never played a banjo before? But we also discovered that we had way more versatility than we'd been taking advantage of. I can't speak for what other people discovered about their own abilities—sitar may have been the only instrument Charlie didn't already know—but I learned plenty. I tend not to approach keyboard as an improvisational player, and it turned out that I could improvise okay. Zach had never played guitar in our lineup before, and we learned he's really good at it. At the end, with him on lead and me behind the drum kit, the band wrote two new songs. Together. Which had never happened before.
I don't have a great name for that mindset, beyond maybe "creative goofing." But I think it's one of the best things you can do, either as a solo artist or in an ensemble.