There is still a TON of space left in the banks, so music length and complexity is virtually unlimited with what the Metroid engine has to offer.
Regarding the absence of melodic themes, you could be on to something there. This soundtrack is challenging from a creative standpoint, and has been making me go out my comfort box for sure. I have been taking a more organic, ambient approach to the soundtrack, though I am used to writing more conventional pieces that are melody driven, and it's been getting me into some trouble at times. I seem to be having success when I take melody out of the picture. So, I think the challenge is finding a way to incorporate a melodic theme into the development an ambient piece, or perhaps even building an ambient piece around a melodic theme.