I play just about every instrument that has a string, and it usually worked that whatever was the new one, in particular the new TYPE of instrument, was the one that opend something up for me. Worked that way for mandolin, fiddle, and five-string, but not so much for uke, autoharp, or dulcimer, now that I think of it. I suspect it may also have to do with whether the instrument has some major built-in limitations, like the latter three do. (Before you flame me, I know the limitations are simply "challenges" to the good player, but you have to admit that things like sustain, limited chords, high pitch, and a gapped-scale fret pattern do make it harder to play as wide a variety of music as some other instruments take "in stride.")
Anyway, sometimes a new instrument will call up a heretofore buried side of your musical personality (like Leadbelly and Blind Willie songs on a 12-string) simply because of the affinity of the sound with the source. That is, you "hear" 12-string for Leadbelly, so you don't really expect to "sound right" till you get one, and all the actual knowledge you've possessed, which you haven't tried to use 'cause it didn't "sound right," comes out in a rush, and what a rush it is!
Not that I ended up suddenly channeling Bill Monroe when I bought my mandolin, but I did widen my "canvas" of available sounds when I took it up. The result was that, once past the learning curve, I could play better on both guitar and mandolin than before taking up playing the latter.
Another aspect is what a new instrument of even the same kind (i.e., six-string guitar) can do by way of making some things easier, or louder, or better-sounding. Most people report enjoying playing more when they first acquire a new guitar/banjo/mandolin/fiddle/whatever, especially an "upscale" one.
Enough blather from me. I've had this experience many times in the past, and expect it to reoccur whenever I get another instrument (which is about monthly of late).