Seems like a nice idea. Maybe have people to list their offers, so you have this sort of localbitcoins essence to it. but if you want to do shapeshift and localbitcoins together then it doesn't really work. Shapeshift is a centralised exchange, the exchanger has its own stock levels. But your site will have people listing their offers on the site and your site does sort of an escrow thing. Not only will this result in at least a few orders being not fulfilled due to scammers and the buyer having to be refunded, which is a hassle, it would also mean the potential of dirty funds being used to exchange for bitcoins, since the buyer cannot select which one to buy from.
Also i'm not sure if you are aware but there are many other exchangers like this already but none of them are doing a shapeshift sort of implementation yet. Look at changer.com, rchange, and goldux.
I mean UI like ShapeShift, which i love it. So simple and straight forward.
There are many exchanges out there, but i selected 'bank' as a niche and want to go deep in it.
OK lets see how it goes!