Good target would be to have every functionality Twitter has. Design could be very close to it as well so new users (Old twitter users) would feel right at home.

Only after that should you guys start thinking about what fancy features could you build. Now the roadmap is laid out to you and you just have to build it.

This was also my advice to Steepshot team. We don't need to reinvent the wheel. Just by using Steem as backend, the product will be better. This way you'll get your product out to the markets a lot faster and users will adopt it quicker as they already are familiar with the product.