In a interview, bungie talked about the idea of user created content and replied that they actually love the idea but is not something they are working right now. Possibly this could mean that bungie is not thinking about forge or custom games, but that its certainly soemthing that they like to see in the future.

Forge world was so much fun! Destiny looks rather serious, I love serious, but I also like to be goofy and play fun games with my friends.

We don't know, though I would find it highly unlikely.