4: Hyper Training
Did you know Pokemon have hidden "effort values" (EVs) and "individual values" (IVs) that affect their performance in battle? EVs can be influenced in a number of ways. By battling certain Pokemon, it will increase a specific EV. For example, defeating a Geodude will increase the "defense" EV. X and Y introduced Super Training as a way for players to min-max EVs in a specific way without having to knock out a bunch of Geodudes, and Sun and Moon has even more ways: Festival Plaza and Pokemon Pelago.
But, until now the only way to influence IVs was through birth, meaning if you wanted to get the "perfect" Pokemon you had to take a crash course in eugenics, breed a shittone of Pokemon together and hope for the best. Sun and Moon introduces Hyper Training, which allows you to max out a Pokemon's IVs by trading in a rare item. The item is rare enough that it isn't something you can do willy-nilly, but it does eliminate one of the most annoying factors of creating a perfect competitive Pokemon.