Manga-themed mangas? As opposed to… what exactly?
Anyway, I wouldn't mind seeing him do a more futuristic series, especially seeing as how he enjoys such things. It'd be pretty fun to read as a result. Wanted! might make a good series as well.
If I could pick though, he'd stay in the Odaverse with a short series that's entirely from the Navy perspective. Not even necessarily with a set cast, but sometimes devoting a chapter or a few to this Marine or that. The Buster Call officers alone could give him several volumes to work with.
One way to go about it would to run a lengthy section about somebody like Smoker and then break it up with... I dunno, T-Bone or even Nezumi before returning to a larger story arc about... say... Garp or Aokiji. Every so often, he'd throw in a story about somebody outside of the Navy, but still in the World Government. For instance, what the rest of CP9 was up during the undercover mission in Water 7 with a chapter that shows a day in the life of _______.
I doubt that would have much appeal to most people, but I think he could get some pretty good stories out of it.