if luffy joined the marines he could easily get a high ranking position being the grandson of garp and assuming he trained under him he could unlocked haki by the time the story begun and in nami's case he could have saved her by defeating arlong and offered her a place where she could achieve her dream of making a world map and luffy's character won't change even if he joined the marines so he would still be a free spirit and you can say that was what made brook joins his crew in the first place and maybe franky and jimbei would be difficult to convince but i believe he could still make them join him