[dj-wonk]

I'm not sure asking here will get you the answers you need. Talk to lots of people, feel their pain, build an MVP. That said, I'm happy to give my opinion.

In my experience, there are plenty of excellent software tools, on the surface. If you want to innovate, I think you might want to think broader -- about how GTD systems actually succeed or fail.

If you were to ask me about the weakest links in GTD software, it has less to do with the "core" software and more to do with behavior. Challenges I've seen include: (a) Forming the habit to use a new GTD tool; (b) Continuing to use it even when life gets in the way and you revert to another system (such as using email, ug); (c) Making it appealing to return to after a break, knowing that there will left-over cruft in there.