In the first place, the Gotham star was quick to say to ComicBook.com that the crossover would need to happen on the Fox series, which would presumably mean that any timeline hiccups caused by Barry’s time-travel shenanigans would be tethered to Gotham and wouldn’t carry over into the Arrow-verse.
That distinction, I believe we all know, is the main reason why Barry should ever be able to use time travel again, since it never, ever, ever goes well.
Let’s push aside all of the understanding that Gotham and The Flash would have to jump through some major hoops in order to make a crossover happen — although that Fox and Disney deal is a similar kind of unprecedented scenario that helps crossovers happen — and let’s just focus on why David Mazouz’s idea is cool.