The biggest thing imo is that the verse needs to have at least an easily-accessible vaguely-dedicated Bible of important information. I'd love to write more in-canon work, but I struggle to even know the names & domains of the greater demons or the relative technological level of the world, let alone finer details.
The two scales listed are solid, although I'd tweak the mental one more to focus on intensity and not personality. Like, when I'm looking for fics(I can almost assure you I'm the person who prefers the lightest works here), it wouldn't matter if the protagonist became a different person if they still retained their humanity.
Like 1 imagine would be little change, 2 would be have identity changes but not otherwise 'normal'(any sort of basic genderswap would go here), 3 is when things start getting 'bestial' and bordering on whether they have humanity, 4 would be more subdued sex beast(for a common example, when someone turns into a mindless pet), and 5 would be pure there's-nothing-left-except-burning lust(any sort of story where someone becomes nothing but a pile of genitals).
I don't really think you need an list of basic transformations though. Anyone actually interested in writing here will have an idea/s already in-mind, so it won't really matter overall.
In any case, the other biggest issue with introducing people to this community is how hardcore it is. Obviously this is kind of baked into the fetish but there's so much more that can be done besides turning characters into nearly-inhuman sex beasts.
Also, there's this community's flagrant masculine-centric homoeroticism. Like, I'm not really complaining because I'll never write something that doesn't involve girl-girl dynamics; but the number of stories involving female-identified characters turning into ridiculous hypermasculine caricatures or mindless sex beasts far exceeds the opposite. Hell, there's a literal sub-setting completely dedicated to female-to-male transformation.
Anyways, in terms of marketing I don't know how much can be done in the first place. What got me into this stuff was seeing one of demon-man's old comics(the one about the mother and son who turn into the two-headed feminine sex snake thing) posted on /d/, and I would imagine most people found Belial during the 'Golden Age' through similar means. Since most of the artists have circled the wagons and mostly not made anything with wider appeal(two-headed sex snake=bizarre but still retaining enough sexuality to be interesting to fantasize about, a giant hairy anus with legs=not so much), that avenue has been mostly closed.
I think a series of lighter, less-intricate stories could be worth looking into. I know I'd be willing to contribute to that if there was a concerted effort or contest.
Anyways, I forgot where I was going with all this so I'll stop here.