Legendary Creatures/Planeswalkers Matter Dominaria may support artifacts and sagas, but legendary creatures and planeswalkers are definitely the main focus of the set: not only do we have legendary creature support (Arvad the Cursed) and planeswalker support (Oath of Teferi) but we even got a brand new type of sorcery, the legendary sorcery (Primevals' Glorious Rebirth), which require you to have either legendary creatures or planeswalkers
Casting the same spell over and over is powerful, but we love variety, right
The more you have, the better I can usually do on price
The Swarmlord Big dumb creatures might make cool commanders for some players, but Id like a bit more oomph from them
They are also much more convenient to lay as a land drop later in the game than shock lands
And this doesn't have the modern templating of "return target creature an opponent controls" as you can bounce your own thing