It would be great to be able to send invites to users, subscription levels (or even email addresses) for events. My ideal solution:
1. When creating/editing an event, add an option to choose between "Public Event","Invite-Only Event", and "Invite-Only Event with Requests". Public events would work exactly as Events+ works now. "Invite-Only" would require an invitation interface, where approved users can be added. Only they will be able to RSVP. "Invite with Requests" would be like "Invite-Only", but instead of blocking uninvited users, it will replace the Yes/No/Maybe with a "Request an Invitation" button, and form to plead your case to be invited, as it were.
2. The admin side would show not only responses, but also include the users who were invited, but have not yet responded.
3. Somewhat of a separate request, but having control over the RSVP options would be nice (hide "Maybe", or change the text of "yes", for example).
That's the main gist. Let me know if there's any interest in moving forward with it!