What if they opt to sorta conbo the 2. With one being the focus of in game, and the other post game. Or the other is otption side stuff as you progress.
Finish a gym, neato the local Totem Pokemon is now wil challenege you since youce proven yourself (or vice versa). You dont have to do the second bit, but if you do you will be rewarded with a TM or Z-Crystal or other item.
If gyms return, Z-crystals will end up in post game like Mega Stones. Going through Trial's to get them would be a better post game then we've had for awhile