Agree on the Node/TexturePaint mode marrying, really important. I can imagine a sort of layer system based on the nodes, simplified and easier to access while painting… but there are more things, smaller, that should be done by now, imho.
For instance, I just suggested today on the interface design forum a much needed color selector popup/widget… Blender 2.8 user interface design
Other small things that come to my mind now:
-Brush Jitter/dynamics for color, size, opacity… (really missing those)
-Quicker line tool (should be one click, like grease pencil)
-A button/shortcut to switch the orbit style (makes more confortable painting in many contexts)
-Shortcuts to toggle pen pressure (I change that quite frequently, and we don’t even have the right click option to add a shortcut)
-A quicker way (button&shortcut) to paint trough the meshes (right now we have to go to the “project paint” panel and click on three checkboxes, then check them again to go back)
And the list could go on, with bigger more complicated stuff…