Well, perhaps a plot where the AllSpark from Transformers ends up on Earth and its energy attracts both the Transformers and the Avengers. The Avengers are initially suspicious of the Transformers. But as they learn about the Decepticons' plan to use the AllSpark for evil, they join forces. The Hulk could even have some interesting interactions with the larger Transformers like Grimlock.
A great plot could be that a powerful artifact from the Transformers universe is found on Earth and it has the power to either save or destroy the planet. The Avengers initially think it's a new weapon or power source, while the Autobots know its true origin. They have to convince the Avengers of its importance and together protect it from the Decepticons and any other villains who want to use it for their own evil purposes.
A possible plot could be that Megatron is accidentally sent to a parallel universe where the Transformers have a different set of values. He initially tries to take over but then realizes that there is a greater threat looming. A powerful alien race is planning to invade both universes. Megatron, seeing the potential destruction, teams up with the Transformers against this common enemy. Along the way, he has to overcome his own pride and learn to work with those he once considered enemies.
Well, perhaps in the fanfiction, the Transformers' AllSpark has some kind of energy that can power Ironman's arc reactor in a more efficient and powerful way. Ironman teams up with Optimus Prime. They face a common enemy, like a new type of villain who wants to steal both the AllSpark and Ironman's technology. Ironman uses his genius intellect along with the Autobots' strength and abilities to fight this threat.
Perhaps the Transformers are in search of a powerful artifact in the Naruto world that is rumored to have the power to unlock new forms of energy. Naruto and his comrades join the quest. Along the way, they face challenges like dealing with rogue ninja who also want the artifact. The Transformers' abilities, such as flight and super strength, combined with the ninjas' jutsu and stealth, make for an exciting adventure as they race to find the artifact before it falls into the wrong hands.
One possible plot could be that the Autobots and Decepticons end up on Remnant. The RWBY teams initially mistake them for new types of Grimm. But then they realize they are sentient machines. They team up with the Autobots to fight off the Decepticons who are trying to mine Remnant for its resources.
The plot might involve the Avengers being called to investigate strange energy signatures which turn out to be related to the AllSpark from the Transformers universe. The Decepticons are after the AllSpark and the Autobots are trying to protect it. The Avengers and Autobots join forces. Thor could use his powers to fight off the Decepticons' attacks from the air, while Hulk could take on the physically strong ones. The fanfiction could also explore how the different personalities of the Avengers and Transformers interact, like the witty banter between Spider - Man and Bumblebee.
In a more comical plot, Hagrid could accidentally stumble upon a small Transformer while in the Forbidden Forest. Thinking it's a strange magical creature, he brings it back to Hogwarts. The Transformer, being a bit lost in this new magical world, causes all sorts of chaos in the castle. Harry and his friends have to figure out how to get the Transformer back to where it belongs while also dealing with the daily challenges at Hogwarts.
One possible plot could be that the Titans are actually some kind of ancient, corrupted Transformers. The Survey Corps discovers a hidden technology that has the power to control or at least influence the Titans. Eren, with his Titan - shifting ability, might have a special connection to this technology. The Autobots arrive on the scene, initially seen as a threat by the humans. But as they learn about the Titans, they team up with the Survey Corps to fight against this common enemy, using their advanced weaponry and Eren's unique power.
One idea could be that the Autobots find themselves in Equestria due to a space - time rift. They team up with the ponies to fight a common enemy like a new breed of Decepticons who are after the magical energy in Equestria. The ponies could use their magic to help the Autobots repair and upgrade, while the Autobots offer their advanced technology to protect Equestria.
One possible plot could be that Oliver Queen gets transported to the Avengers' universe. He teams up with Hawkeye, sharing their archery skills and combat tactics. They could go on a mission to stop a new threat that combines the technology of the Arrowverse and the super - powered enemies of the Avengers.