Uhh... This is already a thing? Just go to your profile, click on
'pokedex' (Displayed right next to the boxes) and there should be a
button labeled missing. Click that, and you can see every single
pokemon you are missing. Btw, the poked feature I am talking about
is
NOT the same as the poked feature shown under the 'home'
tab. I hope this clears things up for you! ;)