In a fanfiction plot, perhaps there is a multiverse - related crisis. The boundaries between the DC and Marvel universes start to break down. Batman and Iron Man are among the first to notice the strange occurrences. They meet when they are both investigating similar anomalies. At first, they clash because of their different approaches. Batman is more methodical and calculated, while Iron Man is more impulsive. But as the situation worsens, they put aside their differences. They join forces with other heroes from both universes to repair the multiverse boundaries and stop any villains from taking advantage of the chaos.
One possible plot could be that Batman and Iron Man meet when a new global threat emerges that requires both their skills and resources. Maybe it's a high - tech villain who is stealing advanced technology from Wayne Enterprises and Stark Industries. They start off distrustful of each other, with Batman being suspicious of Iron Man's more flamboyant and open - ended methods, and Iron Man thinking Batman is too much of a lone wolf. But as they work together, they learn from each other's strengths. Batman teaches Iron Man about stealth and hand - to - hand combat, while Iron Man shows Batman new ways to use technology for crime - fighting.
The plot might involve a time - travel element. A device malfunction sends Batman from Gotham to Iron Man's world in the Marvel Universe. Iron Man initially mistakes Batman for an enemy due to his strange appearance and methods. However, they soon realize they have a common enemy who has also been affected by the time - travel incident. This enemy is using the knowledge of both universes to create chaos. Batman and Iron Man then team up, with Batman using his detective skills to figure out how to reverse the time - travel effects and Iron Man providing the technological know - how to build the necessary device.