I pick these characters because of how little we know of them or because of things Oda has left hanging for so long.
1. Blackbeard. Dude is a D, caused Wapol to flee Drum. No idea how strong he is and we've seen so little of him yet we know he is going to be a vital character.
2. Smoker. Has a message for Luffy, don't know what it is about. Not to keen on the WG but is determined to catch Luffy.
3. Buggy. A flashy bastard, has met Whitebeard before but don't know under what pretext yet. Was in the same crew as Shanks. Now we have confirmation of Shanks' status in the world, their former captain could of been someone big and Buggy holds the key to that answer.
4. Shanks. We know the dude is powerful but just how powerful, we don't know! I wanna know his bounty!
5. Bartholomew Bear. Same reason as Shanks. Has a huge bounty and is a Shichibukai but just what about him makes him hold suh a high position!