###The Problem
There are more and more cryptocurrencies being created every day. A look on coinmarketcap.com yields quite a few, some worth more than others. The one issue with most of these coins is that it's not easy to accept each form of coin as a currency in say an eCommerce application.
###The Solution
Massive Cryptocurrency API would allow anyone to easily accept these coins, throw in a low transaction fee and you've got yourself a nice little money maker.
This would be a Node.js application maybe (well if I were building it, it's what I would use). It would be super lean, secure and fast. The coins would go into various crypto wallets housed on different servers, coins would ideally be moved to cold storage if in the wallet for more than X amount of time, withdrawals would be slightly delayed (like Paypal's 3 to 5 day wait for bank transfers anyway).
###Caveats
Cryptocurrencies are inherently insecure if you're not in control of your own coins. Exchanges are being hacked on a weekly basis now: Coinex and Mtgox are two of the recent ones to fall. A payment API would be subject to attacks on an almost daily basis.
Quite a serious monetary investment for security and secure hosting would be a must. If you're accepting money you're dealing with customers on both sides (the merchant and buyer), if either looses out because of a hack, everyone loses.
There are anti-money laundering laws, compliances you have to pay for and other things to consider from a legal perspective. This means you will also need to pay a lawyer at some stage, as well as fees for registration/compliance, on-top of hosting costs as well.
###Existing Solutions
The biggest competitor to such an idea would be Coinpayments. They already accept a wide range of coins, their site however is very outdated in appearance and when it comes to trust, people factor in the look of your site. You could also undercut on amount of coins accepted, lower fees and better integration options too.