Ok but not gmail. The only way to improve Yahoo mail would be to have it be able to link to other apps in order to make attatchments easier. In short take a look at gmail and all of its features and benefits, then get to work. Your better off designing an app that does something that Google hasn't done already. Be original in your chosen path, don't try to keep up with the Jones's, be bold and carve your own path offering services and Apps that provide a solution to something that's trending.