With last fall's "Invasion!" The CW upped the ante for its Arrowverse crossovers, bringing together the stars of Arrow, DC's Legends of Tomorrow, The Flash and Supergirl to defend Earth against the threat of the alien Dominators. The event delivered blockbuster ratings, and earned praises from critics and fans alike. So, how can producers possibly top that?

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It's a good question, one that's been on the minds of executive producer Marc Guggenheim and his collaborators.

“If we end up doing what we’re talking about doing, it’s going to be pretty cool,” he told Entertainment Weekly. “The crossover is always the most challenging thing we do, but it’s also the most fun. Every year we feel the pressure [to top ourselves]. I feel like each crossover has topped the previous one. Topping last year’s, that’s a high bar, if for no other reason than what do you do that’s bigger than aliens? We’ve got some very, very exciting ideas. ... Very early on in the process of doing last year’s crossover, Greg [Berlanti] said there’s probably no way to get bigger than aliens, so the best way to make the [next] crossover especially epic is if you can’t increase the bombast, increase the emotional stakes and the emotional payoffs, so that’s a very oblique comment on where our heads are at for this season.”

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"Invasion!" was the third annual Arrowverse crossover, following 2014's "Flash vs. Arrow," in which Grant Gustin's The Flash and Stephen Amell's Arrow to fight Captain Boomerang, and 2015's "Heroes Join Force," in which they confronted Vandal Savage. The fourth crossover arrives this fall on The CW.