I think the key question here is - will the next game be 9th gen, will we get another game along side sword and shield, or will we get another expansion?
I think we should get another expansion - this gen almost feels like I'm playing an MMORPG, so by having another expansion, we still can freely travel to the old locations and use those resources, without switching game etc. Keeping all your Pokemon in the same boxes and stuff will be helpful, too!
I'm with Anthrix.
I feel adding more expansions to Sword Shield will be better. Add a new region for Generation 9, and maybe even add in older generations.
But we'll absolutely get a new generation at some point.
Pokemon is the highest grossing media...thing in the history. This cash cow is gonna get milked more than Miltank.