They're obvious, by the "rules" Oda has laid out for the characters. Luffy won't fight without a reason. So he has to meet good guys who are victims to an arc's bad guys. The bad guy has to either be mentioned before the arc or he has to kick some no-names' asses, to establish his credibility as a threat.

And this is an "adventure arc", like Skypeia was. Getting there and exploring was the goal. If they don't meet the good guys and bad guys, they'll just move on somewhere else. Yes, it would be cool if they found treasure or the secret of the Poneglyphs without having to fight some random guy, but I doubt Japanese preteens are very interested in that sort of thing. They want to see fights.

At least Oda is creative about the formula. There are lots of places this arc can go, even if we know it's going to one specific place along the way.