Rather than it always being the player looking for trouble or just walking miles from check-point to checkpoint in quest objectives. I think there should be some on-spot encounter based pop-up objectives between players when in combat with creatures as well while the player wanders. Say an area was all level 30s, kill all of them in fast enough time, and a level 50 deathclaw walks in after the fight, kind of like an unscripted boss trigger or an ambush, and you get a notification to kill the new enemy; kind of like a layered optional survival mode.
Of course it wouldn't be done like that, but just an idea of condition changes in encounters and battle objectives would give more depth and reason to tweak your guns outside of which ones get the instant-kill.

Maybe? The post-game content shouldn't be an issue, if Bethesda carries over the Radiant quests from Skyrim. And I think they've learned to be more creative with what they do with Radiant Story, rather than "Find [item] in [dungeon] for [NPC]" - although I don't much care if there are a few of those quests for each city, since they're easy to make and don't really take away from anything else Bethesda adds.

Well yeah, the problem with Skyrim was that it had too many boring fetch quests as the sub-objectives, rather I think outside of the character interactive quests, the combat related and candid exploration moments should have its own spectrum of on-spot sub-objectives for battle as well in parallel to the fetch & info retrieving quests just so you get something to do in between the travel from A to B. Kind of like say, ambush modes or random area boss triggers with bonuses if you survive.

I think you want to turn FO into something it's not. You seem to want horde mode type stuff which should never ever be in this game. Dynamic questing maybe, some random stuff. But I really think you're expecting to much. Like you're request features

I see what you're saying and it might seem like that because it hasn't been done in a Bethesda game, but I hate that feeling of emptiness when you know how big a world is, yet can barely any variety to really interact with it, within it. Especially combat, where its just: Encounter, shoot em, dead. Repeat. It doesn't really feel like an RPG at all at its core if it plays exactly like a sandbox game in function.