SG1 is also an unusually easy thing to write fanfiction for, especially cross-overs with other universes... because you know, stargate. Not only that, the stargate is top secret, so knock-on effects are limited. The characters fill roles in a kind of nonoverlapping way so they're easy to reproduce faithfully: the dynamic between O'Neill and Teal'c or Daniel Jackson, then there are colorful characters like Rodney McKay, and you can guess what General Hammond would say to any hypothetical situation.

My all-time favorite fanfic is one where SG1 ends up on the Sword Coast (Dungeons & Dragons setting) and encounters such bizarre things as flesh golems and translation magic, but it's executed so well. https://m.fanfiction.net/s/5236609/1/Debt-of-Blood