The folks at DC would simply need to craft a story in which Barry Allen or The Legends use their time travel abilities to go back to the non-specific era in which this particular version of Gotham City exists.
From there, they can encounter Bruce Wayne and get into all sorts of wacky escapades.
So we already had fans who want to see this happen, and now Bruce Wayne himself intends to see an Arrow-verse crossover at some point as well.
It’s a phenomenal idea, although after Flashpoint I’m sure most people would likely want to kill Barry if he ever tried another time travel adventure.