at 1920x1080 the hud floats up from the bottom, is there a way to make it tuck neatly in the bottom and corners like GTA V does?

This game looks dated with this 4:3 looking HUD on a widescreen.

Funny, the game doesn't even display properly on 4:3, 5:4 and similar either. The UI just vanishes off the sides.

Think its possible for them to fix the HUD so it scales fitting in the corners neatly on widescreen? or would that be impossible since its part of the engine?

Probably, seeing what the SKSE could do with Skyrim I'd lean towards yes. The system looks flexible enough. If not, people are already working on F4SE (Fallout 4 Script Extender) and there'll be a mod sooner or later.