From what I gathered Billy wrote the vast majority of the guitar solos on the records, and James just played them sometimes, trading off with Billy every night as to who played what. At least until Mellon Collie and post-Mellon Collie, which is what I see as the decline of the band anyway, and I didn't like the songs on there James co-wrote. Billy wrote most of the bass too. The Pumpkins are really just Billy and Jimmy, which is mainly because Billy can't play drums. And Billy and Jimmy both worked on Zwan, which was pretty far removed from the grunge/alt-rock style of the Pumpkins, so I'm very speculative.
Much as I would love another alt rock grunge masterpiece the likes of Gish or Siamese Dream, or even an opus like Mellon Collie, I really can't see it happening. The Smashing Pumpkins have been one of my favourite bands for a long time, and I'll definitly see them on tour, but I'm not going to get my hopes up about new studio recordings.