Hitmontop could be the hardest Pokemon to get after Generation 2 releases
Hitmontop is an alternate evolution for Tyrogue, which Tyrogue evolves into Hitmonchan, Hitmonlee and Hitmontop. This Pokemon evolution is very tricky, they could be lazy and just add Tyrogue and Hitmontop to the game and allow you to evolve Tyrogue with candy and have extreme luck when evolving it. Which they should just follow the GBA games to add some rarity to Pokemon in possession.
In Generation 2, we got to see the first few baby Pokemon and breeding alongside it. During the GBA games you would meet a trainer named Kiyo, who specialized in fighting type Pokemon. When you beat Kiyo, he gives you a Tyrogue. In order to evolve Tyrogue into a Hitmontop, you would need to have a Tyrogue with even stats in Attack and Defence, 1 point over and you have no chance at all. Even with the even stats it still only works 60-70% of the time, which is great when it comes to making Pokemon worth having.
The way I suggest they do this is by giving Hitmontop some great CP stats while buffing the alternate Hitmonchan and Hitmonlee in order to increase value for this Pokemon. They could introduce Tyrogue two ways, by special event or by randomly finding Tyrogue through the new tracking system. When they do this they should either eliminate all Hitmontop spawns, or make them very rare. The way you can tell if your Tyrogue has even stats would be just a simple use of the Pokemon Appraisal system.