The Straw Hat Pirates may be one of the smallest crew in the One Piece universe, not counting the Straw Hat Grand Fleet, but that doesn't necessarily mean they are lacking in friends. The Straw Hat Grand Fleet is the prime example of that. There are also weird cases of Luffy inviting unique characters to join his crew, as well as those who rejected his invitation or can't join him due to various reasons. Among those characters, there are a few who the fans immensely wish to see become one of the Straw Hat Pirates.
Vivi and Karoo
Luffy and the gang met Vivi and Karoo during the earlier days of their journey. Vivi was still part of Crocodile's Baroque Works then, and she and her partner Mr. 9 were tasked to assassinate Laboon for his meat. The Straw Hats stopped their plans and proceeded to Whisky Peak, which turned out to be an island filled with bounty hunters. That's also the time when Vivi's real identity was revealed. Upon being saved, Vivi asked the pirates to fight and help her free her country, which Luffy and the others happily obliged and accomplished. Unfortunately, the princess opted to not sail with them for the sake of her country. The Straw Hats understood her feelings and left without saying a word, giving birth to the famous hand-raising scene.
Vivi and Karoo are considered to be honorary members of the Straw Hat Pirates, but their presence aboard the Thousand Sunny remains a much-awaited event. After all, Vivi and Karoo are supposed to be traveling with them if not for Vivi's status. The princess has also traveled, fought, and bonded with the other Straw Hat enough to win a lot of affection from fans.
Laboon
Laboon was the whale Luffy and the gang saved from Vivi and Mr. 9's assault. It's the first creature the Straw Hats met after entering the Grand Line, but they soon discovered that the whale had self-harming behavior. Laboon would constantly hit the Red Line, causing its head to be covered in scars. Crocus explained that said behavior was the whale's attempt to cross the seas, which was impossible due to its size. Luffy then challenged Laboon to a fight. After piercing the whale with Going Merry's mast, Luffy declared the match a draw. He told the whale he would be coming back for a rematch and painted a sad attempt of a jolly roger on the whale's head, telling it not to erase it until they meet again.
Laboon's salvation was a heartwarming scene, and its reunion with the Straw Hats is yet another anticipated event. Fans aren't just looking forward to Luffy and Laboon's rematch but also towards Brook fulfilling his decades-old promise t0 it. Of course, the whale traveling alongside Thousand Sunny is what fans are looking forward to the most.
Bon Clay
The Straw Hat found an unexpected ally in Bon Clay, also known as Mr. 2. They first made their appearance while the Straw Hats were en route to Alabasta, catching the crew's heart with their ability. The Straw Hats quickly got along with them and were left in utter shock after discovering their real identity. However, the friendship they built during the short period of time wasn't fake. They impersonated the Straw Hats and sacrificed themself to let the Going Merry slip through the Marines' grasp. Bon Clay proved this once again during the Impel Down great escape. They accompanied Luffy across the levels, and once again sacrificed themself to open the gates to Luffy's freedom.
Bon Clay's sacrifices undoubtedly earn them a place aboard Thousand Sunny, and Luffy will certainly extend an invitation to them the next time they meet, especially since Mr. 2 has repeatedly proven the true meaning of friendship -- a genuine, wholehearted concern to one another.
While fans are more than satisfied with the current crewmembers of Straw Hat Pirates, wishing for one or two more characters to join them is inevitable. On top of that list are a handful of individuals who shares with them a couple of precious memories. Hopefully, fans will get to see at least one of these characters sail the seas of One Piece with the Straw Hats.