piggy-backing a bit an old post by Kyle Gann, if we're living in the guitar age it may be because the guitar spans so many popular styles in addition to "traditional" forms in concert music. There's no reason either the guitar or the banjo need be straitjacketed into any expected style.
Mark Sylvester's music is worth checking out, and he's written chamber music featuring the banjo as a concert instrument.
Speaking as a guitarist one of the great obstacles in the way the guitar can be approached is basically conceptual. This can seem particularly prevalent in a field of guitar activity where it shouldn't seem to be, classical guitar. The idea that somehow sonata form is not amenable to the six string guitar seems ridiculous if you've familiarized yourself with the works of Sor, Giuliani, Diabelli, Matiegka and Carulli. Now whether you enjoy their handling of sonata form as much as that of Haydn, Mozart or Beethoven can be a whole separate discussion, but sonata form isn't "that" hard to engage on six strings. I'm sure it's possible (and may well have already been done) that someone has composed a sonata allegro form for solo banjo. If there hasn't been there should be. :)