The GP expressly mentioned the only selling point of Mint: "automated balance and expense tracking," Mint imports data automatically without any user action. Unless your software does this, your audience will be extremely limited.

Budgeting software has been ubiquitous since the dawn of personal computing, that's a solved problem.

I'm using GNUCash. I used to manually go through receipts and check against bank transaction history. That wasn't exactly sustainable so I wrote a python script to take the CSV that most banks will export and spit it back out in a nice and consistent format for importing into GNUCash. I could potentially explore the scraping capabilities of GNUCash, but with my current method it reminds me to pay down my current credit card balance while doing the exports.