I think there are two really good ideas here.
The first idea is for the HBD calculation of "pitch 1-5 combined goodness" to differ based on role. My current understanding is that the "combined pitches value" takes all the pitches into account. P1 is weighted most heavily. The other pitches contribute less and less. A P5 can be of some value, even at a relatively low number, as a show-me pitch. The formula is the same for all pitchers, regardless of role.
I propose that the calculation be changed so the "combined pitches value" be calculated differently for starters and short relievers. Starters are essentially penalized if they don't have at least 3 good pitches (in the real world, fastball, breaking ball and change up, but in HBD any 3 pitches will do). Having a fourth and fifth pitch is of significant value.
For short relievers, however, only the first two pitches are of any value. The other pitches are not used in the calculation.
The calculation for a pitcher need not be one of the other. Depending on the pitch count set for a pitcher in the Player Settings, the formula could slowly move from being based on a starter's repertoire to a closer's.
The net effect would - hopefully - be neutral. Starting pitchers getting by on only one or two good pitches would be penalized. Starting pitches with 4 or 5 decent pitches would do better. Similarly, closers with only one or two good pitches would be rewarded because the calculation would be based on only the pitches they'd thrown in real life (real life closers aren't throwing their 5th best pitch), while closers that had been taking advantage of 5 mediocre pitchers would be punished.
See my new suggestion for the second idea.