So not sure you could always get a reward whether you have every pokemon in your pokedex but how about getting rewarded for doing so? Maybe another MasterBall? Shoot maybe a rare candy? A voucher to get cheaper items at the store, i dunno. Just a suggestion. Also if you do add a pokedex from the other gen's perhaps have a reward for finding every pokemon across all three gens? Shoooooooot do whatchu want<3