I think in 'Marvel The Flash Crossover Fanfiction', the most interesting part would be how the Flash's abilities are perceived in the Marvel universe. Since the power scales and the overall feel of the two universes are different. Maybe the Flash would be seen as an anomaly, or perhaps a new kind of hero that the Marvel characters have to adapt to. It could also explore how the Flash's speed affects time and space in a world that already has characters like Doctor Strange who are well - versed in such concepts.
The Flash is definitely from DC Comics. DC has developed a rich and complex storyline around him over the years. He's got his own unique powers and adventures within the DC framework.
The Flash is definitely a DC Comics character. DC has built a rich story and universe around him. He's got super speed and is a key figure in many DC storylines.