I think next time we meet Buggy, we will have some idea of his level of strength. Several things point to the fact that Buggy will be a major threat on the future:
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His fight with Luffy was never that serious, and the fight ended before Buggy could show his full powers. Besides, it was the only major fight so far where Luffy was directly helped by another character, and at some point Buggy was not even paying attention to Luffy anymore.
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Oda chose to keep him around. That's pretty much all the reason I need to think that he will be a major threat.
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He is the only villain so far that knows Luffy's weakness (the hat)
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He is surviving the Grand Line. You know, the place where are all those pirates, all those marines, and even cities have protectors (like Galley-lah or Pell & Chaka)
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He has some connection to Shanks and Whitebeard.