Well, the games are going to be set in a water-logged tropical region that's (likely) isolated from all of the others. Sounds a tad familiar, doesn't it?
And, also...
What happens when you take May and give her Brendan's dark hair and a red version of his odd cap? :P
And, while it hasn't been confirmed, the Sun/Moon theme easily lends itself to an Endless Day vs Endless Night conflict centered around dueling evil teams. What was the only other region that had dueling evil teams? ;)
GSC don't count because they were RBY sequels, and Gens 3 and 4 don't count because the Kanto/Johto stuff was largely limited to remakes--RSE and DPP had few-to-no throwbacks or references in them.
Besides, you can't entirely compare the Hoenn callbacks in SM to the Kanto callbacks from XY. XY were made almost a decade after FRLG, so Gen 1 wasn't exactly fresh in mind anymore when they came out. SM, on the other hand, are
directly following Hoenn remakes, so the region is still very fresh in mind and has already gotten a ton of promotional attention. (Especially, when you consider that ORAS dominated Gen 6 marketing in a way that FRLG and HGSS never did in their respective generations.) After all the Gen 3 nostalgia that Gen 6 had, do we really need another generation of it?