The Stan-Kyle relationship has honestly stagnated in recent years. Kyle is spending a lot more time getting tied up in Cartman's schemes and antics.
Meanwhile, Stan's most memorable developments in recent years have been about personal problems, as have Kenny's, so even though we don't really have much development in the Stan-Kenny relationship, there's a lot of fuel for angsty fanfic ideas and concepts, especially when there's Kenny's deaths to take advantage of too:
- Stan falls into substance abuse, gets Kenny hooked into it too, and Kenny dies from it
- Stan falls into depression and starts undertaking risky behavior just to try to feel again, Kenny tags along and is killed
- Stan outright attempts suicide, Kenny intervenes and dies saving Stan
- Stan gets hurt saving Kenny from death, leading to immense guilt from Kenny since he would have just come back anyways