I believe the episode in Sun & Moon that features Brock & Misty and a few of Ash's old pokemon may have been testing the waters a bit for the future, as they did say that if the episode was well recieved; they'd consider bringing back other old characters,Also, you guys think Journeys will bring back some of Ash's old Pokemon? Even if it's not the ones at Oak's Lab, some of the released or in training Pokemon, (like Greninja, Lapras, Primeape, Butterfree, the Alola Pokemon, etc) would be nice to see again.
And just recently we had the return of Korrina, so i don't think that'd be difficult to do. Especially since i recal them mentioning that every single pokemon ever would get shown off in the series.
Though if they went the route of Ash returning to Oak's lab, it'd be a cluster of pokemon.