The Abyss, a plane filled with infinite layers of demonic hordes. Yet it seems to me that they're forms aren't as diverse as that might suggest. So if you want a CR 9 demon, you're choices are Vrock, or advanced lower level demons. So my suggestion is having Type I - IV demons and then using a menu system like Astral Constructs to have various different types of demons.
What do you think? Is this just silly? Is there a better way to go about it?
WAAAAY back in an early Dragon magazine (and reprinted in Best of Dragon 1) was a table for generating random demons. It was pretty good for the time. Of course, this was for 1E.
I haven't looked at all of my various D&D monster books recently, but I would be surprised if there weren't a few more demons at the CR9 level. I think I counted somewhere close to 30-40 different demons that have been published in one edition or another of D&D (to say nothing about second-party material).
The argument (in Faces of Evil and Fiendish Codex I) was that the Abyss is all about Darwinian struggles for survival, so certain tanar'ric types are more common for others because they've been proven to be the most efficient. This isn't necessarily entirely about speed and endurance and big claws, though that's important, but about the terror a given shape inflicts due to its notoriety.
Most tanar'ri don't have unique shapes, although any of them could potentially evolve themselves that way through sheer will, because the first thing a demon thinks when they see a unique form is, "That's new. I wonder if I could take it? RAAAURGH!" while if they see a vrock they think, "Yikes! A vrock! I can't take that!" or "Oh, a vrock's not worth my time to fight. I'll order it to be my minion or be on my way."
That is to say, if you live in the Abyss and you don't look like anything else, you're going to constantly have to prove your power level to everyone you meet, while if you take an established form you might be able to survive a little longer on reputation alone. That's why only demons of lord-level power typically have unique shapes.
Plus, there's a huge industry in the Abyss called the Blood War where balors, mariliths, and nalfeshnee create armies to fight in it in huge batches from larvae they channel through Woeful Escarond; because they're all created by the same nalfeshnee judges, who have found certain forms useful over the eons, those types the nalfeshnee judges create (armanites, rutterkins, babau, etc.) are going to be common throughout the Abyss.
That said, there are hundreds (at the very minimum) of Abyssal layers, and there are going to be uncountable numbers of rare and exotic tanar'ri shapes evolving to fit them. Fortunately there are uncountable numbers of WotC and third-party monster books with demons in them already. Enough for me, anyway, and it's easy enough to come up with variations on the fly (for example, an aquatic dretch, or an armanite adapted for marine or arctic climates, or a balor with the head of an ox, or a marilith with a tentacle attack, or a glabrezu with the head of an eagle and six snake heads instead of hands). That is, use the existing demons as your "types" and just describe them differently enough so your players don't notice they have very similar stats.
There was a random demon chart in the 1st edition DMG too, by the way. It evolved into the hordling chart.
While there are indeed standard demon types, I think there would be quite a bit of variation that could be found in the Abyss. The realms of chaos are going to value the forces of evolution and natural selection, and evolution requires a pool of mutations to work on. In fact, I'd go so far as to say that the nalfeshnee and other higher demons would make a point of forcing smaller demons into different forms, to provide such a pool within their forces. They wouldn't try anything systematic, but rather employ the primal chaos to induce random mutations in the basic forms, and then pit these against the forms already extant in the Abyss. Most will fail. The rare successes pay off the investment.
Demon lords will differ in their estimation of the value of this pool, of course. Baphomet, Prince of Beasts, is explicitly described as producing all manner of half-breeds, grafts, and other forms in his Tower of Science. The yochlols might have been created through this process, or perhaps Lolth as a deity is more capable of producing new forms by design, while demon lords rely on the unpredictable strength of mutation and testing.
As for the danger an unusual form faces in the Abyss, the logical response is to make the mutations not immediately dramatic (creation of a whole new form at once, like the yochlol, would require divine understanding of the entire desired endpoint and all its ramifications). Most mutations would be small variations which, if successful, slowly accumulate over time.
These considerations give us several options. To make a more powerful demon, one that has an ability which has allowed it to survive, you can use the SRD's section on Improving Monsters. An individual demon that has not advanced through changes in form might have taken class levels. An unusual specimen of a standard demon type might merely have rearranged a few points in its attribute array, or have a nonstandard skill set suitable for surprising PCs with. (If the PCs are cocky about knowing the demon types, this is an especially fun thing to do, as long as you've previously made it clear that not all demons fit the standard molds.)
An excellent source of variation that does not increase level might be Unearthed Arcana's traits (features that give bonuses in some situations and penalties in others), or flaws (a negative feature which, if taken, provides the opportunity to gain another feat). For example, the players might come across a bulezau with Quick (+10ft base speed, but -10 HP), or a gang of rutterkin that are Slow (-10ft base speed) but skilled at Close-Quarters Fighting! These are not particularly visible, so they would not attract immediate attack in the Abyss. If a sharp-eyed high-ranking demon decides that these creatures are more successful than others, it may begin making more of them deliberately, for its purposes, and eventually they might become the standard form.
It's a good idea, I think but lots of work, considering the outsider type's tendancy to use a lot of SLAs. On the other hand, typed demons also give PCs an idea about the strength of their foe and such.
One equitable compromise between the two methods is to use templates, grafts, and mutations along with changes in description to make members of the same caste seem more different and appropriately chaotic while keeping recognition and diversity.