If we're talking about ship fantasies I think it'd be amazing to get a third timeskip where the main cast is reuniting once again. Kyle comes out of the car with two children, one is a brunette, the other a ginger. Hugs Stan, Kenny, and they catch up. When they're doing a toast for their new lives, Cartman walks in, bringing a couple of bags with him. Kyle berates him for being late and Cartman replies that he was buying something for the party. They start arguing and everyone is caught between rolling their eyes and laughing that their dynamic remains unchanged despite the years. Suddenly, Cartman leans in towards Kyle and you hear a smooch off screen, complete with the shocked reactions of the cast and the kids audible disgust. Suddenly Kyle looks at the time and say they gotta go because they have an urgent engagement, Cartman grumbles but accepts to leave the food with everyone. As the Kyman family rush out of the door, Stan and Kenny gaze at each other in shock. 'Did they just- "Stan starts, only to be cut off by the FINAL SOUTH PARK CREDITS. This will never happen it's just funny to imagine the show joking with the idea KYMAN CANON until the very end.

But if you mean realistic stuff then I unironically think an absolutely godtier ending for me would be Kyle killing Cartman. The show ending with Cartman feels poetic and fitting to me and Kyle's life wrapping up with his death, too...
Another option that I think would be sweet as hell would be something akin to the Stand By Me ending lol...I just want the main 4 to stick together...