I know Delta gets it's fair share of hate, but being honest, I liked it alot if only for the fact that they explored the other civilizations spawned by the Protoculture, and the enemy was a near peer compared to the former series having no chance of victory against ___________ but hey, here's a song so we're all friends now.
That said, I think they have developed the song theory quite enough. It's only taken 40 years but we have successfully gone from the Minmay Attack to music being the ultimate weapon. I'd love it if they did something completely different. Hell, a side series like Gundam 0083, 0080, WitP or Thunderbolt where it focuses less on the mythos and more on the harder edge stuff would be awesome.