I'm curious to hear other's thoughts on they think would be useful.
Please don't hold back ideas! There's very little harm positing silly or unlikely ideas, but there is some potential value (perhaps more than you realize).
Inspired in part by calebo's comment here
One thing I'd note is that Guesstimate took quite a bit of time (maybe around 1.5 years engineering/PM time.) I'd guess is that if you were to try to pay for someone to make something similar, it could cost quite a bit (>$200k.)
It may be possible to start with simple Python library with some visualizations or similar.