So, the only thing CC licenses don't provide that the OGL did (besides the opportunity for abuse, I suppose .. lol) is partial licensing. The OGL does allow for Product Identity (specific names, content, etc) to be specifically marked as not coming under the open content licensing clauses.

I think there's an "easy" solution to this though.

Basically do what the creator of Ironsworn rpg did. Release your actual books under CC BY-NC-SA, so that fans can make non-commercial content using all of your material, and then release your core terminology and essential expression of your system as an SRD under CC BY, so that commercial products can be made that are compatible with your product.

It's not that much more work than an OGL, though it does require you to actually remove the names and content you want to yourself, instead of just being able to do it easily like you can with OGL.

Although, I think I think it would clearly be possible to say something like, "All of the text of this work is CC 4.0 BY-SA, but the pictures are not included under this license," so I don't know why one couldn't essentially do the same for a product identity-like remedy. Basically say something like:

"Parts of this work are under a CC 4.0 BY-SA. The following material is excluded from such licensing: trademarks, registered trademarks, proper names (characters, deities, artifacts, places, etc.), artwork, maps, and trade dress. All other material is released under a CC 4.0 BY-SA."