They already have this. Go to the main screen as if you're doing a batch delete of multiple senders' threads. Select the same way (can select just one). Go up to top right corner. Click the dots. Select the per contact option. Once there, select the notifications option. Change sound, flash color, icon color etc there. :-)

It's there. From main list click and hold profile picture then click the 3 dots in the top right corner.