Some things that would be helpful:
1) Minute-level plotting for backtest results, even if only for very short time-period backtests (1 day?) would be extremely helpful in developing and optimizing algorithms.
2) Email/SMS notifications on trade execution, or some kind of daily trade summary for both live-trading algorithms and paper money algorithms. Notifications could be a freemium feature, i.e. subscriptions that give you a certain quota of notifications per day/month with a free tier. Possibly Slack integration too for teams? =)
3) Some kind of source control integration, so we can work on algorithms locally using our own familiar dev environment and push to Quantopian to backtest and deploy. Also facilitates collaboration.
4) A way to run backtests faster. I'm sure serious traders would be willing to pay to use higher end VMs to execute backtests faster. Although there's a fine line to walk between making paid versions faster vs crippling the free tier. I'm a bit ambivalent about this one but it's hard for me to imagine that it'd be sustainable for Quantopian to offer completely free compute for backtests forever.
5) Sharing backtest results publicly outside of Quantopian. And allow us to decide whether or not to share the code as well. This will facilitate your most enthusiastic users trying to get you more users through network effects. Right now the only way to do this is to take a screenshot of the backtest page, which is a rather tedious hoop to jump through.
6) More data! Historical data for stocks that have existed for longer than 13 years would be great so we can establish more confidence in our backtest results.