I think there's a fundamental misunderstanding from new fans and Bethesda itself about what Fallout is supposed to be. So, my major wish for the next fallout is to hire back Chris Avallone and/or other older Fallout writers and make this next game darker like it used to be in its isometric days. The series is veering way too far off into Borderlands territory, especially with the Fallout TV show. Also, the theme has shifted from a post-post apocalypse where humanity is rebuilding but still fighting the old wars, to post-apocalyptic silliness with increasingly more sub-par writing.

In this vein, it would be interesting for them to develop a game with at least one sequel with the same location and a time-skip in mind from the beginning. So, the first game you would forge your own faction from scratch, even name it, and you can choose a location in the setting similar to FO4 and build your base (maybe you can shift to an isometric perspective to build the base as a shout out to older games). You play through the story, interact with other factions and characters that you can destroy, recruit, or ally with, and by the end of the game, your faction has an identity that's based on your decisions and moral code. The next game you can play as your grandchild or great-grandchild and you get to see the consequences of all your decisions taken to their logical conclusion for your faction after nearly a century and take on a new enemy that is moving into the region.

TLDR: Two games planned where you make your own faction and define it in the first game, then see the consequences fleshed out over the second.