To me if something can be put into Pokemon Bank then it is possible to be transferred up to a new gen. The Pichus and Pikachu exclusive stuff were mainly to hype up the mascot in a new form. However there was nothing special about them at all except the cosplay Pikachus having a different move, but those moves were primarily used for the contests those forms were present in. At least with the cosplay Pikachus they came with their hidden abilities which was nice.
But Dusk Lycanroc is different, as well as Ash Greninja. These Pokemon have different abilities, stats, and different possible restrictions applied to them. Why go through all that effort in coding and making them different than normal if they are restricted to a gen?
To compare: Spiky Eared Pichu: nothing special but having spiky ears.
Cosplay Pikachus: different appearances, and a different move for each form.
Lycanroc Dusk and Ash-greninja: different abilities, stats, and certain restrictions, as well as special animations in the form change/the eyes changing in attack animations.
Personally though I don't really like that the anime can just "create" special Pokemon for whatever reason, be it for popularity or otherwise. Why not just keep everything within the main game?