Great work! Microcredit, e.g. Oikocredit, Kiva or Lendahand would be a nice addition.
With regards to the opt-in/out discussion I think it's best to make it opt-out for new users, so by default they benefit from the interest. However, it would have been nice to ask current users to opt-in, since some might have joined because bunq doesn't invest.
I agree with David that it would be nice to be able to see what it is exactly that bunq is investing in.