There are mountains of information about devils and demons all through D&D history and I applaud anyone who can make coherent sense of it all. To the authors credit, I think the article does touch on a large number of iconic ideas.
One of the minor points for me is I might not include The Blood War in my games, so it would be nice to know it could be taken out without a major rewrite for every fiend. And while I really like it, I imagine not everyone's campaign settings will include a barrier to demons and devils entering the Prime Material plane either or perhaps even a PM plane at all.
A few specific points:[sblock]1. Telepathy with Tongues is a major power and it might make sense to turn this into a kind of Universal Tongue that has different alignment shades, but is understood by every Outer Planar. As understanding this language in whole is to understand any other language, it might be mitigated by creature Intelligence, INT rolls, and, optionally, alignment again. Perhaps at high levels it turns into Life Speech or Death Speech or whatever.
2 Followers aren't just for PCs. Even demons and devils can, and usually do, have coteries of followers. Even demons ruled by strength commonly have a lord and pawns they think obey them. These are who are summoned or gated in. Summoning being a form of Astral Travel that doesn't include the material form (at least from the PM to other planes. It may work differently onto the PM). Summoning is more limited than Gating due to its re-usability. Gating is bringing your troops into a markedly hostile environment and all deaths are permanent. Still, this leads into epidemic issue in #5 below.
3. Demons are as interbred a bunch as it comes and creating a strain or two as one's followers just makes sense, if you want to breed in loyalty to your pawns. I think this is likely why many Demon Princes have trait carrying followers, but there are certainly other options too. (It just has to do with the mass scale).
4. Wouldn't it be cool to outline a small portion of a single devil's playing of the game of demons? How it connects to so many others, the long game and short game of misdirection and nuances, ever scheming, ever second guessing, never trusting. Even a picture this web of relations perpetrated by lies would be interesting as a DM example.
5. Devil Summoning may be due to allies in like bands being those one would call in for aid. This makes Devils more dangerous due to the relative strength of their aid, but the ultimately the power of the summoning spell needed is what balances this ability. However, it might be interesting to have devils being able to Gate like allies, but that quickly becomes an epidemic of plane ending proportions. Add in the Upper Planar reactions and most any contested plane becomes a battlefield eventually won or loss one way or another. But if the Deities have a pact halting Gating into the Prime Material, then interference is by Calling from PM inhabitants only - as you already mentioned as the status quo.
6. I like LightWarden's point in the article's Comments section. It would be nice if these two forces were less uniform in identity.[/sblock]Suites, allowing for large numbers of abilities[sblock]Idea: Design creatures with several Special Abilities and Spell-Like abilities, but don't allow them to use every single one every day. Allow each, say, 12 with 7 usable for a day. Or 7 and 4 in a day and so on. We don't even need to limit it to a day. Longer times work too.
Spell-Like abilities already function like spells and can be changed to this system easily. Special Abilities are typically permanent and at-will, so they would always be available. What I suggest is moving more powers from the latter to the former.
1. This makes manageable the number of abilities a DM needs to account for in any single combat or encounter.
2. It allows powerful monsters to be very powerful in the strategic scale game over days, months, and even years as antagonists - not to mention the opportunity for growth over time.
3. It enables creatures to customize their abilities to the challenges they face. For example, powerful demons may select abilities to specifically counter and overcome those of the unique PC party it faces.
4. It operates like the magic system already in the game. At least in that many options are available, but not all Spells are Spells Prepared during any given conflict.
Demons & Devils are high level foes for the most part and the majority have a lot of abilities and resources to call upon. Think of them like high level PCs. They are very capable in a lot of ways. They have acquired many treasures, have many allies (well, henchmen), have learned many strategies, have developed many powerful abilities, and generally may choose from a long list of options with which to challenge the players with. They tailor themselves to the challenge shoring up their weaknesses and playing to their strengths. This improves not only replay value, but a depth of strategic design for these figures to be major antagonists and players in a variety of fields, not simply solely in combat, magic / technology, or on cultural playing fields like politics and religion.
I'd really prefer if high level foes could be more difficult and complex again, but I do understand the need for quick, easy play too. I think the above is one means for a fair compromise.[/sblock]