I would say that most of the "loopholes" that you're referring to aren't really accessible to the average Joe because most of us make what's considered "Earned Income". That's what your salary is considered and it's taxed according to your tax bracket. Wealthy people will structure their income and their holdings in ways to minimize the amount classified in the earned income bucket. The extreme is when executives take $1 salary.
The AI is going to be superior in terms of explaining which factors played in your return. This will become clearer how powerful it is when the application suggests ways to optimize your situation and you basically have a playbook in hand to use.
Sounds like you're going to be using some weighted statistical methods in there.
I also read that you're going the Java route. Decent option for handling the load (free tax returns will pull in a big amount of people - assuming they trust the service), but considering you guys are going to be 4-5, perhaps python with the data packages(numpy et. al) might help with AI (which reads like intense probability calcs).
It would be fun to see everyone claim deductions on dog food the same way a rich guy might deduct on his horses.