You pick which Vanguard card you want to use in a match, with each one coming with unique modifications to rules such as increasing or reducing your maximum hand size, or granting you additional life when the game starts
There are a lot of Espers in the series, after all, so to make them all white/blue/black would be hugely restrictive
As for the best of the new creatures, O'aka, Traveling Merchant turns your excess counters into carddraw, and Maester Seymour grows creatures of your choice as you grow him
Magikarp is the most obvious example, since that was created as a very weak Pokemon that evolves into the much more powerful Gyarados
In comparison to modern eras like Sun & Moon, Diamond & Pearl's top cards (with available prices) can't compete with these modern marvels
And with the unseeded packs, youre more likely to strike lucky than you would with the booster box