The following events would be useful:
Template Create Event
There is Remove and Update, so Create is logical.
Template Use Event
Knowing when a template has been used would be nice.
Plugin Uninstall
To complete the set - called before the plugin is uninstalled (so the plugin itself could listen and do apropriate tasks).
View Comment
This one might not be accepted due to possible performance impacts, but as not all comments are shown (thresholds, personal preferences etc) it would be nice to know when a comment is shown.
Confluence Started & Confluence Stopping
To be published after confluence starts and before it stops. See CONF-4022.
Plugin Started & Plugin Stopping
I've not checked on this, so it might be already possible. I'm assuming that a plugin that is enabled and installed will not get any plugin events during startup (or shutdown) ... I would like this to change so that the plugin knows when it is started (i.e. its classloader has been created and the plugin is loaded successfully). Same for stopping.