I absolutely love Upheaval. It’s a good balance between the crunchy complexity of Pathfinder and the ease of use of 5e. There are some very cool mechanics to it, too. For example, the weapons are amazing. Each type of weapon has its own traits; hammers are better at destroying objects. Swords have expanded critical range. Big weapons can be used to intimidate people, etc.
But the most fun part is the customizability of it. Warlocks have an invocation system similar to the old 3.5e setup, and have an eldritch form where they can transform into a very customizable “beast” type of form. Fighters have a ton of highly customizable abilities and a stamina system to recharge them in and out of combat. Monks have combat stances that enhance them, and there’s an elementalist class that basically lets you be a bender (as in Avatar).
There’s tons more. It’s some guy’s personal passion project, and it’s just him developing it so it’s a little rough in places, but it’s definitely worth a look.
Oh, GLaDOS personality in something like an elder brain. Sign me up.