I would love to learn Guitar and Banjo.
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I currently play clarinet/bass clarinet (well I know how to play it but don't anymore), drums, bass guitar, and guitar. My piano skills are pretty bad, so I'd have to say that the instrument I'd want to learn the most is piano. Next summer, I plan on getting a decent keyboard with saved money and whatnot to practice on.
With piano, I could write songs easier, play a wider range of things by myself, and incorporate it in my current songs to add a new depth and layer of sound.1