(SPOILERS)
After the credits roll and the screen turns to black, the usual mid-credits scene begins! A vault opens to reveal The Infinity Gauntlet! A hand reaches in, "Fine..." It is revealed to be Thanos, "I'll do it myself." Thanos now has The Infinity Gauntlet, or so we thought!
According to Kevin Feige there are TWO INFINITY GAUNTLETS! Thus, the one "Thanos" grabbed was not the real Infinity Gauntlet. Also, that might not really be Thanos. If you remember back to "Thor: The Dark World" Loki is still alive and has replaced Odin on the throne. Loki could easily walk into the vault and steal the fake gauntlet, but why would he be impersonating Thanos?
Thanos says "...I'll do it myself." So if it is Loki he could mean that since he is free to roam in Asgard he would just take The Gauntlet himself. "Thor: Ragnarök" could show Loki's return and with Thor searching for answers he could come upon Loki's return and stop him once and for all! The fake gauntlet would be retrived by Thor and he would have even more knowledge about the Gems and the real Infinity Gauntlet.
It's just a theory, but it makes a lot of sense and would explain a lot. With Loki on the loose and Thanos showing no sign of doing anything it would just make sense for Loki to have grabbed that Gauntlet. Although if it was Thanos, by saying he will do it himself, he could mean that by Loki and Ronan failing to retrieve the gems and deciding to take the Gauntlet himself. It could be seen either way, just a theory. We will just have to wait and see.