Additionally, one could use an observer to fire at a notifier point which exists at the end of a section of core code, and that observer could do something "else" and replace whatever values were prepared by the core code with whatever the (addon's) observer wanted it to do instead. This could let you completely replace whatever the core normally does, or simply augment what normally happens, or completely replace the results calculated by whatever happened.

Again, without very specific details, it's hard to guess what exactly the right solution is for whatever you're dreaming up