Have a look at Armory's ArmorPaint, a powerful open-source 3D painting tool with a number of great features. 

ArmorPaint is a standalone tool meant for physically-based texture painting. The tool provides instant visual feedback in the viewport as you paint and then you get textures for PBR-capable engines such as Unreal Engine and Unity.

It's a node-based toolkit which means that the workflow can be quite flexible. It also supports brush nodes that can be used to create painting patterns and procedural brushes.

ArmorPaint was designed to run completely on the GPU, giving a smooth 4K painting experience (up to 16K textures).

The team also wants to add ray-traced baking, painting effects, and viewport rendering. You can learn more about the open-source tool here.