This amazing new creature is not only a Shapeshifter that can take on any other type of creatures, which is essential for any tribal deck in standard, but also a Changeling, which makes it possible to chain several copies of Realmwalker in the same turn, if they happen to be on top of your library.
The two most powerful tribal decks in standard right now are Giants and Elves. You could also throw in Cats and Goblins into the mix for good measure. But the first two archetypes will definitely need all four copies of Realmwalker in their main decks.
Another great possibility is the Party mechanic decks, which could definitely fit a copy or two of Realmwalker.