So basically like a localbitcoins.com 2.0. It would work in a similar way, with some additional features but the main difference is that it would support not only bitcoin, but a few other cryptocurrencies as well, like 7-8 currencies, the most established ones.
Do you see a demand for such platform? Would you use it?
This is an interesting idea to run with because peer to peer sites are still few at the moment compared to exchange sites but what I would suggest as an addition to what others have given is to try and focus on some region for now and push such to a successful point before thinking of expanding. The areas to focus on should be regions where its difficult to trade bitcoin as I have seen some threads about people in some countries that are lost about exchanging their bitcoins for fiat and vice versa.
Another to be concerned about is what to do better in combating scam and resolving conflicts as these two major issue will continue to crop up the moment you are dealing with people and when you do it better, you tend to spend less on advert as people who have used the services will be the one doing the advertisement.
In the case of advertisement, try to ensure that it is directed at your specific audience in that region talked about above and you can then run with that. Wishing you all the best in making bitcoin penetration better.