SS will allow you to travel to all previous regions. All pokemon will be obtainable except mythicals and version exclusives. You can fight all previous gym leaders, E4, and champs and you even visit the interpol jail to fight all previous villain groups and leaders. You can even fight all previous protags a'la Red. Since that's gonna be a big level curve they'll increase the level cap to 200.
You can visit all previous battle frontier, tree, mansion and all other postgame stuff and SS will even give a new frontier twice as big as the emerald one was with 15 attractions.
Every starter pokemon will get a mega evolution
we'll get eeveevolutions of every type
triple and rotation battles are back and they'll also introduce war battles where you get to fight 6 vs 6
Also all the mythicals will no longer be casually given away but instead you will get an item to cause an in game event related to said mythical just like the tickets in gen3
we can also have 3 save files now