Despite D&D usually failing to fire up any of my plot juices, after re-reading some of the demon lords fluff I've started envisioning a campaign in which Baphomet and Yeenoghu spill their forces onto the material plane for a grudge match to end all grudge matches, with the realms of mortals being caught in between the steadily growing tides of clashing demons.
Initial stuff is small scale, an increase in the number of ghouls roaming the countryside, a few gnoll warbands popping up where they shouldn't be, minotaurs and beast cultists clashing with the former, minor demons starting to join the forces as time progresses, minor settlements being raided or outright sacked until eventually each amass an army and assault castles or cities to establish a true foothold in the world and create permanent gates into the Abyss from which the demon hordes can march.
I imagine the players role initially being hunting down some of the smaller bands of roaming monsters, foiling the odd cult or quarantining a ghoul outbreak, with them gradually finding out what's actually going on and having some opportunity to try and stop or slow the creation of mortal monsters and the summoning of demons, or possibly them deciding to throw in with one of the demon lords.
Eventually being tasked with helping evacuate villages in the path of the hordes or sack them depending on allegiance, prepare defences and ultimately try to keep demons from capturing major cities long enough to prepare to strike at the sources of them, or join the vanguard of the hordes in routing the other.
Regardless, I'd rather like a clash between the PCs and Baphomet and/or Yeenoghu, possibly at the same time in a 3 way brawl.