THe dream of Franky is not to sail, its to build the ship of dream. Franky joined to be sure and see his dream ship fulfill his dream. Technically, Franky's entire arc revolves around the notion of pride and being able to keep pride for the "child" until the end in bad or good situation. So Franky joining the crew to see the ship is COMPLETELY predictible as long as you look closely at the thematic. This is without mentionning all the others clues.
DUDE... COME ON. --'
Yeah you are in complete denial. This is pointless. THe drawing litterally has all the attribute of Franky, the Ace ventura hairs, the glasses, the weird nose, the big forearms, the naked legs, the height, the post. But yes.. its "too bad" to be a simple foreshadowing...
The system actually showed that Momo didn't have enough to join the strawhats compared to Carrot. And the system changed compare to the time Carrot was a candidate.