I want to create a startup only focused cloud accounting and metric software company. Will it work? Is the market big enough?
Recently launched Subleger http://subledger.com/ is trying to do some or all of what you describe. It doesn't mean that there isn't room for others but consider that many early-stage companies don't have complex revenue in-flow so the core of what you're describing (converting or merging income into other operational metrics) might not have a wide appeal especially for startups.
Hopefully you get some good answers here but really whatever anyone (myself included) says here is far less valuable than asking startup CEOs what their financial painpoints are with respect to reconciling their app's internal metrics with revenue and expenses.
Finally, the question "is the market big enough" is too open-ended to answer for you. Big enough for what? To attract significant outside funding? Maybe not big enough. But to build a great income for you and a few others? Perhaps!
Happy to discuss this with you further to help you in your evaluation of the opportunity but as I say, best thing to do is canvass the potential market first.
As a CPA/Chartered accountant that's worked in a Big Four Firm, run the finance department at a venture back startup, as well as run my own businesses -- I can confidently say that the only accounting concept that matters for a startup is cash in the bank.
Specifically, how much is there and how quickly is it running out. Everything else really is a distraction as it doesn't matter: when a ship is sinking, you don't care about GAAP compliance.
Metrics matter more for revenue than expenses, which is why this forces the market to be for later stage companies. And those later stage companies have multiple offerings from the mature accounting and management reporting market.
I mentor startups in the product management process so they can make better decisions.
Quite frankly, you are asking the wrong questions. What accounting problem are you going to solve for startups? Without knowing this and verifying it with people that would be potential customers, you can't possibly know whether or not your vision of the solution will work.
I will go one step further and warn you that feedback you get on here about whether or not it will work should be taken with a grain of salt. Always beware of second hand customer validation information. If you need some help getting first hand information, I expect that I can help point you in the right direction.
Cheers and good luck!