Looking back on the "Invasion!" crossover, it's clear to see "Arrow," "The Flash," "Supergirl" and "DC's Legends of Tomorrow" were firing on all cylinders as they came together to fight an alien menace. The crossover event was also loaded with enough references and characters to make even the most obsessed fan blush.

Still, while there was plenty to keep an eye on in the crossover, there's even more that didn't make it into one of the "Invasion!" episodes. From guest stars to surprising combinations, there was plenty of material left on the cutting room floor. Luckily, executive producers Andrew Kreisberg and Marc Guggenheim are spilling the details about what you didn't see during the crossover.

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Lynda Carter on Earth-1?

At the end of the crossover, a new President of the United States in installed. If you were hoping for "Supergirl's" Olivia Marsdin (Lynda Carter) to do the honors, you almost got your wish.

"Actually in the original draft of the 'Legends' episode, she was the Vice President who became the President -- and the studio had what we all considered to be a very fair note, which is, it was a bit too confusing," Guggenheim reveals. "You get into the parallel earths of it all, but she's not the President on our Earth, she's the Vice President... In the midst of, you know, time travel, and aliens, it just was one -- as Oliver would put it -- one sci-fi problem too much."

Roy played a much bigger role, originally

While "Arrow" fans were happy to see Roy (Colton Haynes) in the hologram sequence -- along with Tommy (Colin Donnell) -- he originally figured into the episode in a much bigger way.

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"Obviously there were moments with Colton and Colin in 'Arrow' that were scripted that we obviously just couldn’t do because of Colin and Colton’s availabilities," Guggenheim says. "For example, Roy was going to be Thea’s boyfriend -- that hadn’t changed -- and they met when he stole her purse, and that also hadn’t changed. I just thought that would have been fun and nice to see."

Sara and Alex -- a match made in the multiverse?

Though they haven't met, there are shippers out there that want to see Sara Lance (Caity Lotz) and Alex Danvers (Chyler Leigh) get together. And it just so happens that almost became a reality, sort of.

"There was a little exchange between Sara and Kara (Melissa Benoist) that I really liked -- I don’t think we even filmed it -- where Sara says, 'Hey do you want to get a drink when this is all over,' and Kara says, 'I think you wanna meet my sister,'" Kreisberg says. "Just the idea of starting the Sara/Alex shippers going ... It’s those little moments that wound up on the cutting room floor."

HR and Speedy also nearly shared a moment

HR Wells (Tom Cavanagh) was in the crossover but he didn't have a lot to do with the assembled superheroes. As Kreisberg admits though, maybe that's because he doesn't add much to that particular situation.

"There was a funny scene between Speedy and HR where she says, 'So what are you doing here? What value do you bring?' He’s like, 'Well I’m writing a book,' and she’s like, 'Oh that’ll be a big help' and walks away," he remembers. "It was all of those little tiny moments."

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An alien conspiracy cover-up is happening

After a large-scale alien invasion, you might be wondering how the people of Earth-1 will react to learning they aren't alone in the universe. As it happens, they'll never find out -- shadowy government figures are keeping it under wraps.

In another moment that didn't make it into the crossover, Guggenheim says, "The character Glasses, we call him Glasses because we're so original -- is he basically says, 'Cover up what? A dozen, you know, weather satellites falling out, you know, falling out of orbit?' Because people see the ships, but no one ever really saw the Dominators."

Supergirl almost took a trip on the Waverider

Supergirl had a lot to do in the crossover, but there's one place she didn't get to visit: The Waverider. Stay tuned, though, because Guggenheim still hopes to make it work.

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"There was a Supergirl moment on the Waverider that I really wanted to get in there," he teases. "I’m not going to tell you what it is, because we may do it next year."

With so much that simply didn't make it into any of the crossover episodes, it's easy to see the shows' writers are not short on ideas. That'll be useful if they expand to a full four-show event next year. In the meantime, we're keeping in mind that there's another crossover coming this season -- and it's going to be very musical.

Posted by:Chris E. Hayner

Chris E. Hayner is equal parts nerd, crazy person and coffee. He watches too much TV, knows more about pro wrestling than you do and remembers every single show from the TGIF lineup. You may have seen him as a pro-shark protester in "Sharknado 3." His eventual memoir will be called "You're Wrong, Here's Why..." TV words to live by: "I'm a firm believer that sometimes it's right to do the wrong thing."