I'm not talking about jobs that were needed. I'm talking about Luffy recognizing something special about a character and then him being determined on making them join regardless if they're gonna have a specific job.
When Zoro had that little thing going with the little girl who gave him food, Luffy made it clear he wanted him to be on his crew, also made a comment about him being a good man or something like that despite the rumors and then made Zoro make a tough decision where he had to join him. When Sanji fed Gin even though he couldn't pay for it. Luffy knew he wanted Sanji to be on his crew. He was in a restaurant full of cooks/chefs he could've asked anyone else but he was determined to get Sanji for a reason.
He didn't do that with Brook. He wanted Brook for the unique interesting side, it was a jerk reaction he does with weird ppl he meets like asking the TB zombies to join his crew. Since Brook was a living skeleton and when he found out he was a musician arised the extra want of Brook.
Now that It think about both times with Zoro and Sanji it had to do with food, so I should look out for that.
As for random encounters he's currently in a prison, nobody knew about Amazon Lily until Luffy landed there. Even if we know the next 10 destinations of the SHs it doesn't mean they can't completely go off course or go places in between. Plus we shouldn't be assuming that we'll even get to these places soon. I mean c'mon Fishman island might end up Jinbei status, here about early in the manga and don't see it until hundreds of chaps later lol.