I think I'm gonna try this out with my playtest group and see how it does. Remind me now and then teinedraig, and I'll keep you updated on any problems we spot. However,
Skill Cats Trait Points
4 22
5 20
6 18
...after giving it some thought, I think this is too cheap. Consider that in a practical sense, 1 trait pt. = 5 DPs. The above basically puts an extra favored category as equivalent in value to the Sense Magic talent. I'd want the extra category to be something the player doesn't go for automatically, but sees if it justifies the cost. You can make up those 2 trait pts. spent on the extra favored category by the time you have 5 total skill ranks in the category newly favored.
So I think I'm gonna start with
Skill Cats Trait Points
4 23
5 20
6 17
I may eventually go as far as 4 trait pts. = a favored category, which would take 10 skill ranks to make up the cost of. I'm a bit edgy about that idea, because if you want a very focused character anyway, the extra trait pts. you get for losing a favored category could probably make a fair amount of difference (that's enough to make Outdoorsman, Quiet Stride or Subtle effectively "free"). I don't want to "overcorrect" and have players always wanting fewer categories and more trait pts, that's no better than them always wanting more categories and fewer points.