The original Crossover Earth was a play-by-mail writing game set in a shared superhero universe. I’d played in it while it lasted back in the 80s, and enjoyed it. In the late 90s, I decided to recreate the game, and called it Crossover Earth ’98.
The game was a mix of collaborative writing in a shared universe, and a roleplaying game. Each player created a hero character and a villain character, and submitted one turn a month detailing what their characters would do. Typically the results involved conflicts either between characters or between characters and NPCs. For conflicts between player characters, the player whose character won the conflict would write a short story detailing what happened.
This was a very writing intensive game, and burn-out was common. But it was great fun while it lasted, and it spawned a similar game, Titans of Tomorrow (which is now also defunct).
I’ve uploaded the original Crossover Earth ’98 site, pretty much as is, for your enjoyment. Note that any email link in that site is likely to be horribly out of date. If any web links are still good, consider yourself fortunate.