Originally sinnoh remakes were to be released in 2021. Due to the global pandemic going on they will be postponed to 2022. No mainline pkmn games will be released in 2021

Mega Evolutions will not make a return in DP remakes. As their name suggest they´re not based in Platinum, so the additions the 3rd version made will not be present. This means Giratina will not have a role in the story, but the regional pokedex will be updated with pokemon from gen 5 to 8.

As I said they´re Diamond and Pearl remakes, so no Battle Frontier. In addition Chairman Rose will appear in the postgame, where he would introduce us to the Battle Tower and the dynamax fenomenon.

Some Sinnoh pkmn will be getting a gigantamax form: Sinnoh starters, Drifblim,Bronzong,Lucario,Abomasnow,Garchomp,Electrivire and Magmortar.

As for now it´s not confirmed wether Palkia and Dialga get new forms. Shaymin and Darkrai will not be obtainable in DP remakes. Right now I dont know if they will be introduced in future DLCs.

However we will get some kind of "wild area" in what is known as the Battle Zone