...some small bits may be added here and there and they might add some bits like subclasses or subraces through patches but I wouldn't expect anything major like an expansion with level cap increase and so on.
Yeah, I can hardly see how they'd implement it. If it's width, so more sidequests or something, then suddenly they'd have to account for reactivity as well; get all the voice actors back in the saddle and so forth. That task would be just as costly as a full game; and no one's paying 60 bucks for an expansion that way.
If it's more akin to Throne of Bhaal, and goes more beyod the base game - then that problem arises again - but they also have to account for all the possible endings and origins... It'd be a nightmare.