There is a Stanford-based start up called Indinero.
https://indinero.com/
It does cash flow planning, integration with major banking ala Mint, and maybe some sort of accountant integration as well.
My wife is a tax guru and we've discussed this possibility you speak of many, many times. We even are poised for a product launch ourselves:
http://www.theseedsafe.com
With her heavy-technical multi-entity tax knowledge, and my web development experience, we could MVP something similar in a couple of months. Integrating with the banks and categorizing expenses is something I've been waiting for as a gem though. :)
Think of it, automatic bank reconciliation, budgeting, payroll, invoicing, AND TAX PLANNING. Aka - run your business, here are all of your W2s, 1099's (employee forms), and pre-filled forms for any and all entities you have set up.
Now, the biggest thing here - and I think this is where indinero is getting caught up - is liability. People outsource their tax preparation to Big-4 type businesses like PWC for liability reasons. Unless either the software, or the accountant is CPA certified you are walking on a thin line as a business (but I could be corrected here).
Budgeting, invoicing, cash flow, all the immediate stuff - is taken care of by a slew of web applications. Consider also Less Accounting - http://lessaccounting.com/, which puts a lot of this stuff together.
Really the cash-cow and home-run would be integrating the tax planning into the business. If this could be done elegantly, legally, and thoroughly enough (but also customizable), you'd be set.
Easier said then done. Anyway, that's my take on all of it with a little bit of the market research. Hope this helps.