I want to see how the economy works. I am very leery based on previous Todd Howard comments about economies in Bethesda games. If it is sufficient, I would like to see it expandable to include trading with major NPC settlements, rival trade NPCs, claimable resources (coal mines, iron mines, steel mills, lumber, etc.) and the ability to create a manufacturing base. Yeah I know, it's Fallout not Civilization. I just think it would be incredibly nifty to be able to build and expand the Commonwealth as some sort of industrial baron.
I would like to see something like Falskaar was to Skyrim, meaning a fairly large additional world space with a lot of additional quests.
Depending on the armors in the game (really hoping Combat Armor and Leather Armor make comebacks) armor to fill that sweet spot between the padded vault suit and the walking tank (powered armor). I'd like to see the return of the Recon armor or a mod to do just that. Something a little more modern too maybe.-
Marts Mutant Mod FO4 edition.
Mutant powers a'la Bioshock Plasmids. Completely 100% lore violating and it would be the coolest mod ever.
A cyberware mod that adds visual signs of augmentation to the character model. And of course, usable by the NPCs. In fact, ripping out cyberware parts from the bodies of your slain might be ways to get the goods to upgrade your existing systems.
Visual overhauls to the powered armor. I want my Starcraft Space Marine, Iron Man, and other armor. Along with that, adjust the flight time of the jump jets. A configurable mod that allows everything from "boost to the top of that small building" to "iron man flight." Along with the obvious visual overhauls, I want sleeker, more modern looking powered armor suits that incorporate the existing mechanics. A redone HUD for power armor that is more holographic would be awesome. If integrated lasers or plasma weapons can be rigged into the power armor, I'm all for that too.
A pip-boy replacement with a more modern looking interface. Same with the computer terminals.
But mostly, I just want the frickin' game. I am going to play it without mods the first couple of plays through just to enjoy the game for what it is.