Guardians Of The Galaxy 3: 10 Things We Want To See
1. Adam Warlock
Chekov's gun principle dictates that details in a story should contribute to the narrative and that writers should not make false promises. Ergo, if someone shows a gun, then it needs to be fired for the sake of completion. In this case, Guardians of the Galaxy Vol. 2's tease of Adam Warlock is the gun, and showing him off will be the bullet.
Adam Warlock is easily one of the most powerful creations in the entire realm of Marvel Comics. His overwhelming power led to him playing a significant role in the comic-book Infinity Saga. Thus, his absence from the cinematic equivalent now leaves him in a strange state of limbo. It's uncertain what his place in the MCU will be, but it has to start with the Guardians.
It's hard to believe that James Gunn would choose not to pay off the last movie's post-credits scene, so you should expect to see the anti-hero make an appearance. One role he could play is as the movie's central villain. The Guardians have had a troubled past with its antagonists (lest we not forget Ronan the Accuser), so Warlock would be a good choice.
He's not a villain, strictly speaking, but certainly has the potential to be and could make for the toughest challenge the protagonists have ever faced.