One option that is already proven as an alternate airspeed measurement is to use the AoA vane. On the DH8-300, the AOA is used to feed a fast/slow indicator (looking like a glideslope indication, but on the L/H side of the horizon). It does not deliver numbers but it will be centered when at 1.3 Vs. So it allows to fly an approach at an appropriate speed.
Flap extension speeds need to be deducted via the speed booklet page - when at Va for the present flap setting, one is safe to extend the gear and also a further step of flaps. Apart from that, flying on this was fairly unspectacular. Programming such an indication into a contemporary airliner should be easily done. Unlike on the DH8, it would not necessarily need to be indicated at all times; besides manual selection eg. via a P/B on the EFIS control panel, it might be coupled with the ice detectors and pop up as soon as they are triggered.
Simple, easy to install and almost intuitive to use.