Here are some ideas:
Joint Accounts
Make the process to define the top up limit much clearer to avoid wasting time with the support team.
Reduced fees for premium members. ( 1.5% is high compared to exchange trading, you can check on premium members if a reduced fees generates more trading activity and hence more revenu for you)
Partnership to offer investment service. ISAs and SIPPs have huge FX fees. If you don’t plan this please say that it’s not in your immediate plans. (huge customer value)
Monthly savings in crypto then stocks. Give a public figure on how much net crypto is bought on a monthly basis through this service to build confidence in a growing market.
Credit score in UK. (Clearscore has been bought for just allowing to display credit score for free, you could do the same quite easily)
possibility to import others external debit card. It’s a good way to centralize all your card numbers.
Account summary page with balance in all currencies/cryptos and the total in your main currency
Web portal to view online the same content as in the mobile app.