As I begin to pull together thoughts on my own low-level infernal adventure, I begin by noting the elements I want to put into it. Like most "Old School" adventures, I want this one to be based on a location, most likely a dungeon or ruined temple of some sort. If there are standard humanoid types, they'd definitely be cultists. I'm also considering a back story of an old priesthood where the high priest became corrupted, summoned a lot of little demons to the temple who proceeded to kill everyone else off, leaving the high priest behind. Either the priest is still alive or it's now some kind of undead, but the "pesky" monsters would be some kind of lesser demon. Either way, the BBEG of the adventure will likely be the demon that tricked the priest into summoning him.
I also think that one of the big treasures from this particular adventure will be a collection of lore in a tome or manual that could be used to either summon demons or banish them back to their home. Later adventures could involve groups that also seek the tome, and will target the PCs once they discover our heroes have the tome. The good guys want it to send some greater demon back home, perhaps, while bad guys want it for the power found within it.
I'll start with the One Page Dungeon that I'm creating for this adventure in my game, and use that as a reference page at the end of the module. That way, the end user can use either format for their adventure that they feel most comfortable with.
Okay, that's what I've got for the moment. I'm sure I'll keep cogitating on it for a while, and I'll be sure to share what I come up with.