I don't think we'll ever get a post game thing like Gen II (which was more a second part of the game rather than a post game). That said, that brings an idea for post game content: give us a reason to revisit each town/city/prominent location. You can make it a one time event or something that happens daily/weekly/monthly, but why not give each town/city/prominent location something to give it and/or its people more character or lore. Maybe have a region expansive mystery you have to go all over the map to solve, or have a group of strong trainers appear all over and if you want to battle them you got to look all over the region. When players want something to do post game they aren't asking specifically for a dedicated post game location like the Battle Frontier, they just want something to do to spend at least a little bit more time in the Pokemon world and maybe even have a reason to go to places again.
Well honestly I think Jynx should have gotten an evolution along with Magmar and Electabuzz in Gen IV. They were the "com mon" version of the
fire, ice, and lighting trio and all got pre-evolutions in Gen II, but then they decided to focus just on Magmar and Electabuzz in Gen IV due to the Flint and Volkner bromance (though they could have still given Jynx one, Sinnoh did have an Ice-type Gym Leader who kind of needed more Ice-types in the original Diamond & Pearl). And I still think it should get one before getting a Mega Evolution, otherwise Mega Jynx would have to find a way to compensate against the higher stat Mega Magmortar and Mega Electivire.
We can list Pokemon we wanted to get a Mega all day, but its all up to GF who don't seem so hot on the idea of Mega Evolution now or at least at the moment. GF, we're not asking you to implement them in the story, we're just asking you to make more for competitive. Hopefully if Stars is true, now that they made all the new Pokemon and Z-Crystals are fully implemented, they can work on some more Mega (and throw in a few more specific Z-Crystals and Alolan Pokemon, may I dare even suggest for non-Gen I Pokemon? Also for Mega, give Gen V and VI some; I can understand you not wanting to do it for the current gen Pokemon).