Thanks for starting the conversation @Henrry_Pulgarin. I think that option 3 would be the most difficult. We would have to harmonize HPA behaviour across all plugins, which would be very tricky.
Thus, options 1 and 2 are the best, IMHO. I think that the best approach would be to use the pod-autoscaling plugin as a basis that other plugins can use to provide HPA.
For instance, the pod-autoscaling plugin would provide a base configuration only for the lms/cms(-worker) containers, like it does today. But it would also provide custom hooks such that other plugin maintainers can add their services there. And there would be comprehensive docs to explain how to do that.
Looking at that PR I strongly recommend you do not go down that road. Putting all the intelligence of your plugin inside a configuration entry will make your config.yml file very difficult to maintain. Instead, I suggest you create a custom filter that you make available and document for other plugin developers.