6 Reasons Why Dark Nights: Metal #1 Rocks

That character, and any of their siblings, just can't get used willy nilly so it's an actual surprise to see.

Dream, and the Endless characters as a whole, a group of siblings who embody the eternal concepts of live itself such as Destruction, Destiny, Despair and Death among others, are the creation of Neil Gaiman. Gaiman, in a particularly unusual and masterful piece of negotiation, actually retains a share of controlling rights in whenever the Endless characters are used. And he is particularly protective and sparing of allowing that use.

In fact recently, another DC book had intended to create a new supernatural character named Doctor Endless, who would embody powers connected to the Endless. This character was created without the express permission of Gaiman and given their connection to his Endless characters, it was needed. Ultimately that character, and perhaps the entire project, wound up on DC's cutting room floor as a consequence.

So for Metal to include Dream in a surprise guest-appearance at the end was exactly that: completely surprising.

It also shows that Gaiman, a very respected storyteller among fans, thinks that Metal is just what DC needs.