Some of the side character stories have been resolved….but not all of them. If we're assuming it's going to wrap soon and Miho and mashiro are getting married, why have Aoki and Hiramura get engaged?
I see a wedding in the not to distant future, but it would have to be Aoki and Hiramura first.
There's still room for Takagi and Kaya to have a baby. Having Takagi try to balance a child with work would certainly not be outside the realm of subjects explored in this manga.
We also have to deal with Nakai and Iwase still. I do not expect them to leave Nakai in his current state. Yes, he has become a wretched disgusting person. However, in the past he was the guy willing to sit and draw manga in a blizzard. Much like Darth Vader, there is still some good in him. Yoshida shi will fix Nakai. It's also been hinted that Nakai and Iwase may work together when Natural+ ends.
There's still plenty of room to do more with Fukuda as well.
As much as I don't like him at all, I would not be surprised for one final Nanamine showdown either.
As for Reversi being a Jump flagship...the editor in chief said that, but he had no idea how long the series would last. Only Hattori knows that Mashiro and Takagi are most likely going to end the series short of the two year mark. Do you honestly think of Death Note as a Jump flagship title? I mean it was popular and maybe for it's 2.5 year run it was considered a "flagship", but once it's gone, it's no more a flagship for Jump than Seinfeld, Friends, and Cheers are prime time must see-TV for NBC in 2012. Dragon Ball, One Piece, Slam Dunk, Naruto, possibly Toriko someday...big series that can carry Jump.
Death Note would never be considered a true rival to One Piece if only for the fact that One Piece is going on it's 15th year with insanely high sales numbers. Even if Death Note pulled equal sales to OP for it's 2 year run, it still would pale in comparison to OP. Right now, Eiji is pulling higher volume sales....and Zombie Gun is just getting started. If it's like Crow, it's going to be another long lasting action adventure shonen. Ashirogi are getting closer to wrapping up their series. Yes, the anime will push sales higher, but let's say the series lasts about a year and three quarters. Ther's about 8 - 9 volumes of material. The Death Note anime lasted 34 episodes or so covering 12 volumes of manga. There's not a ton of material to work with in the Reversi anime. It'll last one 26 episode season...maybe a second. Then Reversi is gone and Zombie Gun can become the true flagship for Jump that Death Note / Reversi could never be.