Okay, I just tested it and looks like it already does that (I thought it takes only the tracked categories/tags, but it does indeed include also watched and first post watched), which is great.
I also tested it with the Custom top navigation links - the drowback is that when it’s selected it still highlights Latest and not the new tab.
I’d also love to be able to select this new tab (I’d call it followed) as Default home page - that would save me so much time having to scroll through categories I don’t care ablout every time.
In short I’d love to see that in core - it lould be the solution to the most important feature Discourse misses, being able to see only relevant stuff at first sight.