The biggest no no I see is the modern weapons. A lot of the weapons in Fallout 2 and Fallout Tactics just didn't fit. I liked the style of the weapons from the original Fallout the best.
The only sword that would probably fit would be something like a power sword. No curved shape or anything like that. Katanas really do not fit the Fallout world at all...
Anyway, the power sword should have a lot of lights and wires and stuff. The style should be similar to the super sledge and ripper.
As for how many of them you'd find? Maybe one or two in the whole game. Preferably just one and you'd have to get it from the BoS by doing some sort of quest and the sword is a prototype model.
But anyways.. yeah. no M-16's and junk like that. Not that M-16's are junk but they are ugly, unlike the purdy weapons of the original Fallout.
Edit: Eh, just drop the power sword idea. They probably couldn't get the look and feel right anyway. As for all the other things in the list I agree 100%, they all suck.
I usually like turn based combat but it wasn't done right in Fallout Tactics. The CTB worked alright though.
But anyways, the absolute WORST combat systems would be like Neverwinter Nights or the one used in Final Fantasy 12 where you just set up their gambits and let them go at it... with no further imput needed from you.