The whole Turkey Sandwich of course! But I'm guessing you want to make something smaller than that.
The reason why I never bought the HX Stomp was how gimped the UI was compared to the full Helix. So I would like to see the full HX Stadium UI shrunk down to something more compact. I get the HX Stomp form factor is great, but IMO it could be a little bit larger just fine.
I think the HX Stomp I/O is pretty spot on, I'd maybe swap the MIDI DIN jacks for 1/8" TRS, and obviously USB-C instead of USB-B.
Otherwise I'm down with
@metropolis_4 's list of improvements. I think on the HX Stadium Stomp the starting point should be "this is a multifx unit" first, full blown modeler second.
I feel the smaller HX Stadium is still large for people who are fans of e.g HX Effects. The full HX Turkey Sandwich in my mockup is somewhere between the Fractal VP4 and HX Effects in size.