Changing combat system would piss hardcore Fallout fans about as much as an engine change. Fallout is not a tactical combat game. If you want tactical combat, in a Fallout enviroment, might I suggest Fallout Tactics?
The Party Member combat control box was good, because you had some idea what your allies would do, but nothing was certain. It's a nuclear fuckin' wasteland, nothing being certain is the closet to certain anything gets.
Turn Based worked, because it kept to the Pen&Paper feel, and Fallout was, for alot of people, their first venture into CRPG. It kept to the roots that made RPG's great, simple, not overplanned TB combat, focus on narrative, open-end-ed-ness, and in turn received greatness of it's own.
To move away from those core systems, would be to move away from what made us love Fallout. Another big part of what made it great was the element of suprise. You didn't know you'd be kicked outta the Vault, and, as was earlier stated by Hotel California (Damn good song, by the way), you didn't know the goons were gonna shoot at you.
Planning in TB is harder than PB, but that was what made it so much fun. You could plan combat, but it could easily go wrong, and you'd be in a tight spot.
Overworked combat would also alienate characters who enjoyed talking, sneaking and lying their way through tough situations.
If one area of gameplay was going to receive alot more work than others, making it the subterfuge would be perfect.