A (cumulative) 1 second paralysis effect on all potions.
Makes sense as while drink a potion you shouldn't be able fight, cast a spell, jump, run, dodge...
Great idea, mym. Poor execution, though. Notwithstanding the fact that it's entirely possible to avoid the paralysis effect, it just seems a bit too... contrived. Let me try.
Let's say that an animation was attached to potion consumption - an animation that takes about 2-4 second to complete. This will prevent you from attacking or blocking during a quaffing, but not necessarily from moving at all. Seems reasonable.
And say you chug nine potions in the inventory at once. That would be, well, nine animations you have to sit through once you exit. While this won't necessarily stop the most heinous applications of stacking, it *will* make it a lot harder to do in situations that can't be easily fixed by modding. Combine BTB's Game Improvements with a setup like that, and you've got balance.
Of course, this could be taken one step further (or not, I don't know - we're way outside my realm of expertise right now) by disallowing the player from drinking more than one potion at a time in the inventory by running a check to see if the animation is running (or supposed to be). Or, if that's not possible, then wait to have the potion effects apply until *after* the animation plays.
Granted, I have no clue how within the realm of possibility *any* of this is, but my job here is to come up with wild and crazy ideas - not implement them