My main gripe with remakes is the Pokémon rosters of the NPCs. All games below 4th gen had pretty bad rosters for a few characters and IMO the Hoenn Elite Four was the absolute worst in the series next to RBY's E4.
Since I mentioned mechanics, let's go.
Day/Night system: cosmetic thing. Not featuring it in RSE was a developer's decision (and, IMO, mistake).
Evolution in certain areas: Impractical with Hoenn's map. HGSS didn't feature it anyway.
Hidden abilities (plus extra abilities), more attacks (and pokemon): That's content, not mechanics. Save for the physical/special split, and a few features that were subsequently added to the series, the game as of now plays absolutely the same as it did upon RS's release. The jump from RBY to FRLG is much bigger than the difference between RSE and B2W2.
The game wouldn't have more Pokémon either since the remakes usually feature the same regional dex as the originals. Only exception was in HGSS where you could get a few Pokémon that evolved when leveling up with a move learned, like Piloswine and Tangela. All additions would be made to the postgame, and, believe me, there's a heck of a lot they would need to add because Hoenn's postgame is non-existant outside of the Battle Frontier.
Furthermore, I don't think GF feels RSE is outdated enough. They're still using sound effects introduced in RS. The minisprites in the Pokémon summary screen are all from RS (at least, the ones from the 386 first Pokémon). The core mechanics of the game are from RS. Unless there's something very significant coming around with XY, we won't see further remakes, be it of RSE, of DPPt or any other game.