This exactly. While I really like the idea of being able to play Diamond and Pearl modernized, with their technical issues ironed out, I'm not sure if it would sell copies by itself to have a game that's "just" Diamond and Pearl modernized. The games would have to have something to differentiate themselves from just booting up the old game on your DS (or an emulator). Something to make the games more than just shined-up DP. There should be something new, that wasn't there earlier.
LGPE did not have much (I remember waiting the entire pre-release period for them to reveal what was the game's actual selling point and they just never did), but even it had Alolan forms and Mega Evolution, plus a handful of entirely new moves. It had
some content that couldn't be found in the originals. And of course, all the other remakes had even more.
Plus, consider marketing. They can't generate much Internet buzz if all they do is confirming the existence of features we already know to be in Sinnoh. Of course, they could squeeze a trailer out of the new way the Underground is implemented, and possibly Super Contests, but you don't really generate hype for a remake solely by confirming that things weren't removed from the original. That wouldn't raise any expectations beyond what people had when they learned the originals were being remade. There needs to be something that tells them the games are worth buying, even if they have played the originals.
It all makes me think they have something up their sleeve. The return of Mega Evolution seems plausible, and if that happens, I think even an expansion could be feasible. The following Pokémon in the Sinnoh Pokédex are currently capable of Mega Evolution:
- Alakazam
- Gyarados
- Steelix
- Heracross
- Lopunny
- Gengar
- Medicham
- Garchomp
- Lucario
- Abomasnow
Adding Platinum gives:
- Gardevoir
- Gallade
- Altaria
- Houndoom
- Scizor
- Absol
Granted, this is 16 Mega Evolutions in total and LGPE had only 15. However, that was also among a much smaller overall population of Pokémon. Also, the current pool of Megas notably excludes Sinnoh's starters and legendaries while only containing five Gen IV Pokémon (of which one evolves from a Pokémon from another region). If Mega Evolution were introduced to Sinnoh, I think it would be plausible to expand the pool a little.
There could also plausibly exist some Sinnohan Forms and/or evolutions (and honestly, I'd
love to see some latter-gen Pokémon reimagined in the design style of Sinnoh!). Or new forms of the cover legendaries, lesser legendaries, or Rotom. Whatever additions they bring is likely to manifest as new Pokémon to use in battle, at any rate. Every new main series Pokémon game since Ruby and Sapphire has, to some extent, added entirely new Pokémon or new forms, and I don't expect these to be the games that break that tradition.
All in all: there will probably be
something.