RSVP port to Drupal 6 will witness major features added to RSVP.
You may suggest or sponsor any feature you might see useful.
The list so far is:
* Support for both Event and Calendar module.
* An API that allows other modules (simplenews, buddylist, etc) to interact with RSVP.
* A nice jQuery UI.
* Views 2 Integration.
* Token support.
* Actions & Triggers integration.