I think it would be great to have the possibility to alter the cache id for the simple cache plugin (or even any other cache plugin).
This need emerged because I've to cache some stuff based on geo-information about the user.
Sure it was no big deal to build my own plugin for caching, but it's somewhat of overhead because all I needed was to add a geo-tag into the cache id.
The cache actions module already implements such an hook:

  // Let other modules alter the cache key.
  drupal_alter('cache_actions_panels_cache_key', $cache);

The code for panels could look like this:

  // Let other modules alter the cache key.
  drupal_alter('panels_simple_cache_get_id', $cache);

Even nicer would be to integrate this feature into the cache plugin handling itself.
But to do so I guess we have to register a dedicated "get_cache_id" in the plugin - similar to the other functions (e.g. "cache get").
Before I start integrate such a solution I would like to hear your feedback.

Thanks & cheers
Peter