I prefer the idea of a strong temporary penalty to accuracy rather than having the use of firearms outright disabled. Movement while under the effect of the flashbang should also be partially randomized, with a 1 in X chance the victim staggers off course to one of the adjacent tiles immediately ahead if he has the TUs and ability to do so, where X is = to the number of these tiles he can potentially move into. The field of view of the affected unit should also be totally disabled once the underlying line of sight system is implimented.
Back to the subject of new weaponry, I'd like to see a tactical mortar weapon, at least in singleplayer. In multiplayer such a weapon would evoke balance issues assuming its range were virtually unlimited as it should be, given that most maps feature open starting positions and the first team to play on such maps would always win. Perhaps this issue could be partially negated by some sort of 'deployment' phase which would require a full turn.
As for the specifics, the mortar would be slow to fire and reload, and highly inaccurate, but extremely powerful, featuring terminal damage, a huge splash radius, and virtually unlimited range. It could also be loaded with serveral ammo types, ranging from incendiary, to fragmentation, to plasma and antimatter once the appropriate technologies have been researched. The mortar is large, taking up perhaps a 2x5 space, with each shell occupying 1x2 or even 1x3 tiles. It would essentially be *the* long range indirect artillery weapon of the PHALANX arsenal; one of the few tactical niches that are so far unfilled by it.