Thanks for sharing your thoughts; I think I can contribute a few worthwhile comments.
It'd add fun visuals and a feeling of all-around battle chaos- imho no matter how long would it take to plan a turn, it'd feel more fast-paced than paused real-time in the style of Baldur's Gate.
Of course someone adding new visuals is fine, but allowing multiple battle actions to display in parallel doesn't require new visuals.
I wonder if "sequential" turn-based started as means to overcome technical difficulties (we can only hold so and so many frames, a computer can redraw so and so many pixels at a reasonable speed etc.), but became the norm so thoroughly that it became a convention, and not discussed/experimented with much.
Maybe, but all except the very earliest games for 8-bit computers and consoles managed to animate multiple (>10) sprites on screen at once - so it wasn't technically impossible to animate several fighting characters. It's just that with limited one-programmer budgets, the first cRPGs focused on innovating in the RPG aspects like stats and places to explore.
I think the Baldur's Gate 'pausable mayhem' was just a brilliant innovation that is so superior that it's painful to go back to sequential turn-based combat, at least with large numbers of opponents. JRPGs of the 1980s which used sequential combat avoided that problem by limiting the number of enemies per encounter to a handful.
Balancing: We can issue commands at once and execute them immediately- every 2 guys who shoot at each other get hit, but if someone shoots at a moving target, he never hits- that's one hard to balance option.
I might be wrong, but I think you're imagining some kind of change to game mechanic that isn't necessary in order to play combat animations in parallel. The Interview by Eurogamer gives a bit of an idea how the combat engine works. It's really modular, which let them substitute the GURPS ruleset with their in-house ACELIPS system.
The game tells the combat module something like: "ok here's this character, with this cover, shooting that character with that cover, give me the % chance of him hitting this target this round." IIRC it just iterates through each combatant with a decision what to do and then asks the game engine for the %hit.
My proposal is to not change any of that stuff. The combat mechanic stays the same. The ruleset stays the same. The sequence of attacks and actions stays the same. We just run through all combatants and start an animation appropriate to what happens as the result of that round. Loop that, round for round, until combat is over.
So for example the question "who hits a moving target, where" is answered by the sequence of actions processed by the combat engine. If the enemy began his movement before the player fires at him, then the player is targeting the enemy at the new position - despite the animation lagging this fact.
Sure you might need to change-up the order of animations a bit - e.g. tack-on the death animation after the character is done walking to the new position... but i'd take that over slogging through 70 hours of tedious combat.....
There is one change which this implies which necessarily impacts gameplay; namely the case where
within 'one round' the battlefield situation changes, for example a death, or incapacitating disability etc. to which the player reacts and changes tactics. Losing this ability means losing some degree of fine control of the combat. Only a real Fallout 1 and 2 expert could tell us whether this would break any game situations. If it did, the old combat could be toggle-able as an option.
On the other hand, if we issue commands simultaneously but someone is faster (so "roll for initiative" considered), we have two guys shooting at each other but only will be faster. But do we let know the player about what did he and his enemy roll, before issuing the command? Or we don't let the player know at all? Or we let him estimate how much a chance does he stand (eg. let him know that the enemy is really much faster than the player)?
I don't think this is a problem, since I don't envison changing initiative or the underlying sequential combat model. But not having actually understood the code, there could be hugely wrong assumptions operative here.
Erm. Now i reread that. Yes the change of battlefield situation within a round is something the player can no-longer react to - so you might end-up wasting a shot on an already-dead enemy, for example. Still if the Faltergeist player could turn on parallel combat at will, it'd be quite welcome, i think.
) and wonder why the system You propose wasn't used so often in older crpgs. 
