Can I develop an API which will accept bitcoins and normal currency using existing API's such as coinbase and stripe? This API when used by a merchant should be able to accept both bitcoins and regular currency on his website.?
tl;dr It could be done, but doesn't seem to be incredibly useful as there are already systems that already do this, combined multiple payment methods into one easy to use system/API. (e.g. WooCommerce)
I don't know much about Ruby or Stripe, but the blockchain API is pretty easy to use and has some fairly decent docs. (API Docs) It just requires you to open the web page it specifies using a simple get request which will return a JSON string. Just unencode it and all of the informations is right there. It only requires one call to create the address to give the user. It will forward the bitcoin to the bitcoin account of your choosing. It will also send a callback to your URL of choice which will tell you when they send a payment. Make sure you don't send the user anything until the payment has at least one callback to prevent double spends. Six is the recommended number for high value payments.
I guess you could make an API of sorts where you specify what kind of payment method the user chooses then having it respond with just a single callback that would alert you when the payment complete was what ever the payment method. Which would have the benefit of the merchant using your API only needing to know your API instead of every API. However WooCommerce and other ecommerce tools acomplish this by allowing you to use multiple payment methods. For example you could use WooCommerce with their default credit card payment method in conjution with my Bitcoin payment method.