A Grimdark Alternative Ending for the 1994 Stargate
The following discussion contains spoilers for the 1994 science-fiction film Stargate, although given how the movie is 24 years old I don't know why that's important.
In the film, Ra plans to send an American nuclear warhead back to Earth, enhanced with the type of materials his advanced technology uses (in Stargate SG-1 it's called naquadah). He states this will enhance the warhead's power 100 times and claims that he created human civilization and now he will end it.
Hold it right there. Assuming that the military sent a W78 nuclear warhead through the Stargate with O'Neill, that's 350 kilotons. A nuke enhanced with naquadah to 100x its force would be around 35 megatons. That's around a third more than the 1960s-era B41 nuclear weapon. Mount St. Helens' eruption was 25 megatons of explosive power, while Mount Pinatubo was around 70 megatons. So Ra sends the nuke back through the Stargate and it detonates in the decommissioned nuclear silo that was the base of operations for the mission. Assuming he can remotely detonate it through the gateway, he's basically just caused a volcanic eruption in the U.S. Rocky Mountains, and that assumes the mountain on top of the explosion cannot contain it. After all, underground nuclear tests generally don't disturb the surface that much.
Mount Pinatubo caused a worldwide temperature drop of one degree, and that was with smoke, gas, and other crap continuously pumping into the atmosphere from the bowels of the Earth and spreading all over the world rather than a one-time big bang and, depending on what's around it, a big radioactive forest fire. Assuming the equivalent of Mount Pinatubo, Ra has just caused--maybe--a repeat of the Year Without a Summer in 1994. That would suck if it causes drought and famine in marginal areas, but it would not destroy modern civilization.
That said, there's a more grimdark possibility here. If there's a major nuclear detonation in the US, it could potentially lead to a confrontation with Russia, although I doubt President Clinton would have ordered a nuclear strike not knowing what happened. Assuming the US panicked and launched on Russia and the Russians fired back, you could have a civilization-ending nuclear exchange depending on what missiles are fired, where they detonate, etc. Bonus points if there's a nuclear winter to cause famine and ecological collapse in otherwise unaffected areas.
Ra (or some other opportunistic Goa'uld who notices) could wait a few years or decades for civilization to collapse, show up and pull his "I'm a god" trick on the debased, uneducated descendants of the exchange survivors, and assume control over Earth. I'm now imagining the Earth's surviving libraries as the equivalents of the fresco Sha'uri showed Dr. Jackson that depicted Ra bringing Earthlings to Abydos and the rebellion that ended his rule on Earth thousands of years ago.
Of course, Ra in the movie doesn't actually seem powerful to pull that off, even if post-nuclear civilization is reduced to medieval levels. He's got a couple fighter jets, one spaceship, and a few guards with armor resembling that of Egypt's gods. The basis of his rule is deception, not raw strength. He might not be able successfully take control of Earth, although one of the TV series' Goa'uld with their planetary empires, navies, etc. would.
That said, I could easily imagine him taking control over part of it. Maybe he keeps Dr. Jackson alive using the sarcophagus long enough to see him returning to Egypt and re-establishing himself as god there once more.
How's that for a Downer Ending?