in a theoretical perfect universe where Bethesda actually strives to make the perfect fallout game, I'd hope they hire obsidian to help them write the story, side quests, and companions. It'd keep all the the fallout 4 gameplay mechanics like settlement building, gunplay, and modding weapons/armor. they'd bring back the karma system from fo3 (like needing specific karma for certain companions) and fame system from fonv (for cities/towns that we cant build settlemens in, and safehouses for major factions full of good loot), they'd give us a plethora of different ways to do the main quests like fonv where very few npcs would be essential. go back to voiceless protagonists only so we could have more dialouge options, but having them voice acted would be ok too bc some of the fo4 options were hilarious. keep the robot workbench from fo4 bc that was fucking sick (legswroth). and finally they'd hire someone with a voice like Mr new vegas for the radio bc I loved him so much.

edit: I also hope they bring back a lot of the consumables from fonv like hydra, rebound, steady, healing powder and magazines that might make certain skill check easier or allow you to lockpick/hack one level above what you currently can.