One of the coolest things in the 1e Monster Manual is in the description of the Type V demon:
Each type VI demon has its own name. (Balor is a type VI demon of the
largest size.) Six are known to exist.
When you kill a type VI demon, you’re not just plucking a prize from the DM’s infinite monster grab bag. You are writing the history of the campaign world. That gives Balor and his five cousins a gravitas that justifies their good stats. (On the angelic side, according to the Monster Manual 2, “there are at least 24 solars.”)
As a DM, I’m inspired to think about and name the other five Type-V demons, and guess at their relationships with each other. Each demon can be written large in the campaign.
What other top-end monsters deserve this treatment? Ancient dragons, perhaps, but I get the feeling that they are so solitary that there’s always one more than anyone knows about, so there’s no point of keeping track.
Storm giants, though, merit further examination. They need to be fixed anyway. As a monster species, they have a problem that they sort of share a bailiwick with, and overshadow, cloud giants. Storm giants were a relatively late addition to OD&D: cloud giants were king in the original game, and storm giants were added in the Greyhawk supplement, maybe to keep up with player power inflation.
The other giants each rule over a particular terrain type. Storm giants are not purely sea giants and not purely cloud dwellers – neither fish nor fowl – but 90% of the time they share cloud giant terrain, breathing over their shoulders.
What if they’re not a superfluous giant species but the cloud giant royal family, somewhere between kings and gods? Like Balor and its ilk, we’ll fix the number of storm giants at some manageable number. Let’s say 12 to match the Olympian pantheon. Throw in some siblings and marriages, or just reskin some other family epic: the Greek gods, the Skywalkers, the Lannisters, or perhaps borrow from King Lear, the Shakespearean inspiration for Storm King’s Thunder.
As the aforementioned fictions suggest, a limited cast of related characters gives room for soap opera. A dash of soap opera gives emotional resonance to what could otherwise be merely sound and fury, signifying nothing. Fantasy is a good place for family drama writ large. And nothing is writ larger than the biggest of the giants.