I honestly feel that Sinnoh should get the 3D remake treatment. And not just because it's the next in line in the series to have a remake, either.
A lot of my problems with Sinnoh concerning how dull it became after the initial excitement wore off have to do with the style of the game, both aesthetically and musically. I feel that 3D graphics would improve the look and feel on the environment of the region immensely, make up for the bland design of the region itself. Also, remaking the music with pretty much any other style would make the soundtrack fantastic(even the Gym Leader/E4 battle theme...). It would just make it so much better and so much more repayable to me, which I would greatly appreciate.
Plus, I would be interested to see how they changed the story and incorporated elements from Platinum as well as more modern features. The Delta Episode in ORAS has given me faith that such a thing can be pulled off, too.
In summation, I truly believe Sinnoh deserves a 3D remake the most, in order to fix some of its biggest flaws in my eyes and compensate for others.