I'd say I'm rather experienced in this matter and I'm curious about what you have in mind.
I was headed in this direction before, but redesigning the entire interface by myself was kind of a huge undertaking, not to mention the huge demands on customization from the player base making it incredibly difficult to tailor for everyone. My current approach fits any interface, even if most of that is not perfectly tailored for a controller. ConsolePort is very dynamic in this matter.
I'm wondering if you might be wasting your time by trying to redesign the
entire interface though. If it ain't broke, don't fix it. Some parts are due for different design choices, but far from everything needs to be changed. If your concept is even remotely piggybacking off the current UI, you should probably wait and see what Blizzard has in store for the UI in Legion.
When it comes to gameplay improvements; to my knowledge, you can't really expand on the system without memory editing, which is a ticket straight to the Warden courtroom. Everything that's related to world interaction is off limits and only responds to mouse and keyboard input.
Also, speaking to you as a company, you're not allowed to sell add-ons for WoW. The only thing you could legally sell is the input mapper that translates controller input to keyboard and mouse for WoW to read. Assuming you're not going to publish it for free, that is.