One idea could be the Boys teaming up with a DC hero like Superman, but then realizing his 'good guy' image might be a facade too. Another could be Homelander facing off against Batman in a battle of wits and power.
A great plot idea for 'the boys dc crossover fanfiction' would be to explore how the corrupt corporate world of The Boys' universe intersects with the often morally grey areas in the DC universe. For instance, Vought could try to manipulate some of the DC heroes for their own gain. The Boys would then have to expose this to the DC heroes who might not believe them at first. This could lead to a lot of interesting character dynamics, like Butcher trying to convince Batman that not all is as it seems in his world. And we could also see how the powers of the supes in The Boys compare to those of the DC characters. Maybe A - Train tries to race The Flash and realizes just how outmatched he is.