No, no manga sequel. There's is absolutely nothing left to do in that universe.
I also want Obata and Obha to work on new stuff, but that's bullshit. There is an infinite amount of content in that universe, Obha's writing isn't character-dependent. It's true that L/Light's rivalry was his masterpiece, but in part II he showed he has no problem making a bunch of engaging characters and scenarios around the Death Note premise. (Not to mention Bakuman where he also showed he can keep a million rivalries strong and exciting for 20 volumes)
Basically I'm sure Obha could come up with plenty of new characters in that world which could be super interesting to read about, albeit not as masterfully as DN's pt.I, I'm sure he could keep up the pt.II quality forever. The premise just lends itself to endless stories. It even makes sense in terms of plot: As of the ending, Ryuk is sitting bored on his ass just like the beginning. It is totally logical he would start causing chaos in the Shinigami realm, or lend his notebook to the humans again. In fact, it makes even less sense to assume his life was uneventful.
But yeah:
1. New stories with death notes and humans, easily done via Ryuk's boredom
2. New stories in the Shinigami realm, easily done via Ryuk's boredom. We can explore the world's mechanics, rules, and culture, which were only very lightly touched upon in the original story.
3. New stories with humans but with old characters: L has two decades years of mystery-solving prior to the main story starting. Near has his whole life ahead of him. These characters are a goldmine for interesting writing by Obha, whether there's a death note involved or not.
Basically I agree with you that they should move on from DN, but it is criminally inaccurate to suggest the world doesn't have more interesting stories to offer.
