Here's the Monstrous campaign concept. Let me know if you're interested.
The arena is a glorious battlefield of victory and defeat. But many do not know the struggles of the slaves in the pits waiting for their possible doom every day. The monsters captured and taken from their homes to be put in a cage to fight for their lives for sport and entertainment. Though many of these monsters are mindless beasts and many of these slaves wicked criminals who spilled innocent blood long before their incarceration...not
all of them are...
Welcome to Monstrous. You'll take the role of a low CR monster who has found kinship with other creatures enslaved in the arena. One of your allies, a clever goblin by the name of Ruckus, has revealed that he knows a way to escape. Together you've hatched a plan to not only escape the arena, but to escape the city and get back to your homelands...hopefully in one piece. The journey will not be easy. Guards roam the streets, and monster hunters roam the wilderness, and adventurers can pop up anywhere. You'll need to stay on the run, struggled to find resources, and most importantly, work together as a team to overcome the perils that await you outside. After all, what's a little more life-threatening danger when your freedom is on the line?
Game System: Pathfinder 1e. Pick a monster of CR 4 or lower. You gain racial hit dice and monster feats instead of normal levels up. You can trade a monster HD for a class level by defeating adventurers with those class levels and swiping their stuff. Adventurers are their own class of NPCs that are far more dangerous than most others (especially if you get caught wandering off alone from your party).