I always wanted one, and now I have one. A bubble to modify!
What does it do? It adjusts a freeform shape in all dimensions using 6 triads and an optional 12 manipulators to adjust the weight between those triads.
You can do this manually quite easily – but having all adjusters available at once is so much nicer for quickly fitting a shape to references or just playing around with everything together.

I took @EvanReese ’s advice from his FeatureScript 101 video (thank you!) and played around with a classic build first to figure out what works. Routing curves from 3D points, then splitting and lofting around Z, looked best.
Making the CustomFeature, I started with a basic UI, added the triads and manipulators until it felt right, and then added the three FitSplines going through a total of eight points.
I experimented with the results quite a bit. Having eight points to position, with the option to fine‑tune using adjusters that follow the curve, worked really nicely for me. (The manipulators are somewhat a hack, since we don’t have real spline “weighting.”)
Lofting didn’t work at first, and my lack of understanding and impatience led me to force Gemini 3.1 Pro to replicate my manual build exactly. That worked—but it’s probably not very elegant.
After some cleanup and a few quality‑of‑life additions – symmetry, custom origin, show/hide curves – I was done. Took me an evening, and I’m quite happy with the result.
@Apple : are you running Onshape in there? 👀