Order types: definitely limit orders + opening/closing auctions
Transaction data: csv export
Watchlists: multiple watchlists
Long Options, Crypto etc.: I wouldn’t offer that, at least on the free accounts. Most people would annihilate their money fast and that’d be bad publicity. If you offer to go long options, then displaying the price as implied volatility, showing bid-ask and easy-to-understand measures (theta decay displayed as an Euro figure, for the uninitiated) would be an absolute must.
Futures, CFD, short options/stocks: maybe only allow people to sell covered calls and do not offer futures and CFD? By that, make ninety-nine known for being a responsible broker? For anything else I’d also feel somewhat uneasy about the risk that ninety-nine could blow up in the event of a market crisis when too many accounts go negative.
“Social trading”: in case you offer to publish portfolios, do it in a smart way, with risk measures (Sortino ratio), adequate bet sizing (no trades being published exceeding e.g. 10% of portfolio value in case of stocks, no more than 5% in case of options, no short term options < 30 days to expiry).
Freemium: besides charging for real time data, maybe combine with advanced charting (VWAP, RSI, bollinger) only available to premium customers.
Web interface: no idea, probably all effort rightly goes into the app now. If you ever do this, add links to company’s investor relations websites and ETF information pages (besides the kiid … rather let people see what they are buying by letting them browse the holdings list).