if I can step in with some opinions:
Make a dark themed UI for night-mode and create a stop-loss function, I like the UI except the charts on http://1broker.uk - it is extremely easy to use.
Also, make a trollbox with a reward system - IRC & Slack with tipping-bots is not good enough anymore. A good Bittrex trollbox will make people stay on your site to talk with their friends and that in itself stimulate to a lot more trading.
Basically - shamelessly copy the right side of Poloniex ;) - markets, price-changes etc
oh and - make it so that we can see our P/L and overall portfolio-value in Bitcoin + USD, Euro, etc - peoples local currency.
stuff like that.