The Sony mirrorless cameras used to have some third party "apps" that were pretty cool but apparently are now all discontinued. And what the Magic Lantern team did for Canon cameras (especially the 5DmkIII) was absolutely amazing. I still miss features to this day.

If you are looking for a new camera also definitely checkout the new Panasonic Lumix line of full frame cameras - they are damn amazing. Extremely intuitive UI, tons of features you do not find in other cameras, amazing video (especially with an external recorder - you can do 6K raw video) and built like a tank. And they all use L-Mount which is shared among multiple camera manufacturers with tons of lenses available from Sigma and others. The Panasonic lenses are pricey but also extremely high quality (probably because they also design super high end cinema glass)