Semi-Independent PF OGL Games
There are a couple of games I own copies of that are mosatyly built around the D&D 3.5/Pathfinder 1E engines that I likely woul;d only get to play on FGU because it may take a bit to find players for them.
Two of them, of some significance, are Ponyfinder and Pugmire/Monarchies of Mau. Both have comp[letely nopn-human casts, both have a bit of satiric bite.
Ponyfinder is a darker take on MLP:FiM world-building that is at about the same part of the violence spectrum as a normal Pathfinder game. Suffice to say not everybody gets along and sometimes violence — occasionally extreme violence -- IS the answer, atg least for a while.
Pugmire is about the aftermath of the Fall of Man, where Dogs and Cats have evolved to full sapience. The dogs follow "the Code of Man" with true religious fervor, seeking to be worthy of the attention of the absent Masters by being as close to the ideal of a "Good Dog" as they can. The cats have their own country and their own take on Man, taking it upon themselves to prove worthy of Man's worship were they there. Very intruiging concepts. they apparently play well, and I'm not sure what it would require to get them into FGU.
I don't know for sure how building extensions works, or wther they they would need to be entirely separate modules that use much of the PF 1 rules and mechanics.