I have a few ideas:
1) I'd like to see the system that the Darwin is using allowing you to easily change your settings and having it calculate the appropriate values for maintaining your settings.
2) I'd like a crystal LCD with a blue backlight showing the ohms, voltage, amps, battery level and whatever.
3) Two extra buttons to navigate the menu (instead of using a wheel like the darwin does). In order to switch between different menu options, you would quickly push both menu buttons down and then use either one to turn the value up or down. Holding the two buttons down for a certain amount of time would turn the unit off completely. There should be a setting that allows you to disable the light. The reason for using two extra buttons instead of only one extra button is to keep the "fire" button exclusively for powering the atty, not the menu.
4) Have a MICRO USB charging port on the bottom of the device. NOT a "Mini" usb adapter, but the MICRO format. It should be able to accept the chargers used by cell phones that use the universal MICRO usb standard. This should make it cheaper to deal with charging cables because you'll probably already have an extra charging cable if you need it. The batteries should still be easily removable so the device retains its flexibility, but with the added bonus of being able to plug in a USB adapter and going without waiting for your extra batteries to charge up, if you so wish.
5)Retain the cylindrical form factor! I hate the box shaped mods! The Darwin's flip down atty bridge has issues with things like the MAP Tank or other large atty mods.
6)Since it would be using a MICRO USB port, you could also add in the ability to store
vaping statistics on the device that you can move over to your computer when you charge using your computer's USB port. Being able to study your vaping habits like that would be really cool. Seeing at what time and the lengths you vape as well as at which power settings might somehow come in handy. You could export it as a txt file to your hard drive.
Edit: 7) Keep the same capability to use the batteries that the Provari uses. And keep everything else pretty much the same unless you're going to improve the materials/texture or atty bridge design.