As to my point about the arcane/divine magic divide…
If you have priests using miracles and wizards casting spells, that is a divide recognized for millennia.
It in no way infringes on WOTC’s intellectual property.
Like using leprechauns, will-o-the-wisps, bears, toads, and frogs: these are all common things and the Coast brigade has no rights over them.
Again, it’s not just the divide its the divide representing fundamentally different sources of power (some natural aspect of reality vs. power from gods) connected to eight specific schools of magic, Vancian preparation, nine levels of spells, VSM components, slot-based casting, wizard armor restrictions, cleric weapon restrictions, fireball being a level 3 evocation while invisibility is a level 2 illusion, etc.
All of those combined add up to a specific expression of a concept of a spellcaster that can actually be copyrighted.
There’s a reason Palladium wizards and priests don’t use magic the same way as their D&D counterparts do, why Tunnels & Trolls doesn’t even have priests and their wizards use spell points, why pretty much every non-OGL system has its own unique expressions for their magic systems. It’s because a particular combination of aggregate elements IS something you can copyright.
To quote
https://guides.library.unt.edu/SCCopyright/basics#s-lg-box-14997580 ;
Notably, copyright protects the expression of an idea, but not the idea itself. This should be intuitive: Copyright is designed to protect art not the ideas art is based on. So, just because copyright protects the Harry Potter stories does not prevent another person from writing a completely unique story about a boy wizard using magic to fight evil. The law protect's J.K. Rowling's specific telling of that story, but not all of the ideas that are contained within the story.You can’t copyright a wizard or any of the individual components that comprise a D&D wizard and their magic; but the specific collection of components is something that falls under copyright and Hasbro/WotC holds that copyright (and not just to the SRD versions, but the version from every D&D edition).
Do what you will. I’m just sharing a potential point of weakness that a vindictive WotC could attack a work with that a lot of people are just writing off because 20+ years of the OGL has made the concepts so common within the OGL community that no one thinks of them as unique copyrighted elements when almost everything that actually does include those elements is another OGL/SRD D&D derivative where Hasbro/WotC just has to say “we’re pursuing legal action against them too” and your “but everyone’s doing it so it’s not unique” argument is largely defused.
How does the OGL allows others to make content for your system in a way different from Creative Commons or other licenses? I would think it was a bit more restrictive because the OGL says those others can't write on the cover "Compatible with Chris24601's Brilliant Game System" and can't use your logo or other things (which are legally allowed if carefully controlled in how) while other licenses would allow that.
OGL in this case is just shorthand for any license that grants a license to use your content because who wants to write “the Open Game License, Open RPG Creative License, Creative Commons licenses and various other system licenses open or otherwise” again and again when you can type three capital letters and convey 90% of the same sentiment.
Also, not all restrictions are bad by default. Not letting eight year olds vote in state or national elections is a restriction that I don’t think anyone would regard as a bad thing.
So too, on Freedom of Association grounds I actually don’t have a problem with “if you want to indicate compatibility just contact the licensor first” as a license condition. Really, I struggle to think of reasons why it would be so terribly important for you to use someone else’s trademarked name without their permission other than because you think they’d object to your product for some reason.
If you don’t wish to deal with that condition (I assure you my conditions for use would not be monetary, but rather center around “don’t shit on my setting”) then I’m sure there are other systems with other licenses you can make content for.
That’s the beauty of Freedom of Association; you only have to associate with people you want to.
I don’t want to associate with people who lack the common courtesy to reach out to someone whose content they’re using word for word and take two minutes to describe what they’re doing and ask permission to use my trademarks on their product.