Sure but the "expression" here cannot include game rules or "concepts".I'm pretty sure most of the stuff in the SRD can be, and is, copyrighted, since it's an expression of those game rules; at the very least it's ambiguous, since the line between uncopyrightable rules and copyrightable expressions has never been defined for something as expansive as a tabletop RPG.
So simply rewriting the rules and the concepts in ones own words, is fine usually enough.
Rolling a d20 and adding a "Dexterity" modifier to determine the outcome of success, is an example of something that cannot be copyrighted.In that regard, I suspect that the expansive nature of Pathfinder works against it if it ever came down to a court battle. For instance, having a die roll to avoid a combat effect is probably not copyrightable; calling it a "Fortitude saving throw" probably is.
Yeah, this is stuff that I would doublecheck.Likewise for the issue of real-life mythology and beliefs. Having a monster named a "babau" isn't copyrightable by itself. Having it be a demon that's covered in acid and has a "sneak attack" that deals 2d6 extra dice of damage, telepathy out to 100 feet, etc. probably is.
For example, five different kinds of "chromatic dragons": white, red, green, black, and blue, with specific breathweapons (and sometimes appearances) starts to get murky. But then again, almost every culture on the planet has their own system of assigning specific colors to specific elements, and to have dragons embody an element as a breathweapon is public domain.
For example, the Welsh famously have the battle of the Red Dragon versus the White Dragon.
And so on. Consider, "Superman" can fly (Air), is strong as steel (Earth), has laserbeam eyes (Fire), and supercool breath (Ice-Water). One cannot copyright superpowers.