Based on the information available from the One Piece manga and anime, the idea of the Cross Guild allying with the Straw Hats is not at all ludicrous. In fact, it's a very popular fan theory and has a lot of basis in the series' events.
Here's why such an alliance is a plausible and even likely idea:
- Shared Enemies and Goals: The most compelling reason is that both groups are essentially enemies of the World Government and have reason to oppose Blackbeard's crew.
- Against the World Government: The Cross Guild was formed as a direct response to the abolition of the Warlord system. Their primary goal is to hunt Marines and disrupt the World Government's order. The Straw Hats, particularly after the events of Enies Lobby and their various run-ins with the Navy, are also firmly against the World Government's authority
- Against Blackbeard: It's highly speculated that a major conflict in the final saga will be between Luffy and Blackbeard. The Cross Guild's members have no good reason to ally with Blackbeard. Crocodile and Mihawk are both independent-minded, and Blackbeard's chaotic and manipulative nature would not align with their individual goals (Crocodile's "Operation Utopia" and Mihawk's desire for a worthy opponent). In fact, a major clash between the two Yonko crews (Luffy and Blackbeard) would be a perfect opportunity for the Cross Guild to side with Luffy.
- Existing Relationships and Mutual Respect: The key members of the Cross Guild have pre-existing relationships with the Straw Hats, and some of them are even friendly.
- Buggy: Buggy and Luffy have a long history, starting from the East Blue all the way to their unlikely alliance and escape from Impel Down and Marineford. They have a certain chaotic camaraderie, and Buggy's desire to survive and thrive often leads him to align with the strongest party, which could very well be Luffy in the final war.
- Mihawk: Mihawk has a deep respect for both Luffy and especially Zoro. He sees Zoro as the one who will one day surpass him and has even taken on the role of his mentor during the two-year time skip. This action, training his future rival, demonstrates a unique respect for Zoro's ambition and strength. He is invested in Zoro's growth and survival and would likely not want to see his worthy opponent defeated by anyone else. This relationship makes a temporary alliance against a greater threat highly plausible.
- Crocodile: While Crocodile was a major antagonist in the Alabasta arc, he surprisingly helped Luffy multiple times during the Marineford War. He saved Ace and later Luffy, and while he tried to play it off as having his own reasons, it's clear he has a certain disdain for the World Government and a grudging respect for Luffy. The possibility of them teaming up to face a common foe is not out of the question.
- The "Enemy of My Enemy" Trope: The final war in One Piece is shaping up to be a massive conflict with multiple factions. It's very common for rival groups to form temporary alliances to take down a greater threat. In this case, the World Government and possibly Blackbeard's crew could be that greater threat, making an alliance between the Straw Hats and Cross Guild a strategic necessity.
While the two groups are rivals for the One Piece and would likely never be "friends" in the truest sense (and Zoro vs. Mihawk is an inevitable final showdown), a temporary alliance to achieve a common goal is highly probable and would be a logical development in the story. It would be a great way to bring back fan-favorite characters and their complex relationships with the Straw Hats.
