Yes, the music needs to be more memorable. I don't think it necessary to have Marty to make it sound Halo, but it could be a good thing. They could attempt to emulate his music in the next, which is why you may not actually need the man to make it sound his way.
As of right now Marty's future is unknown. If there was something I could undobutly say about the music in the 5 Halo games he composed for is that you could recognize and say "This is the Music of Halo." He gave different flavors to the spinoffs such as reach and ODST, making them feel as if they were their own game, yet keeping that same feeling as before in the original 3 games. It was magical music, and it was most likely the first thing in Halo: CE that hooked and dragged me in deeper into the franchise. It would be great to hear it again... and not in Destiny.