I'd love to integrate one of the calendar booking systems into Marketo - and then improve upon it.
For example, you'd send an email to prospects before a tradeshow to book a timeslot for a demo. The landing page pulls up with only those tradeshow dates selected (so the prospect doesn't have to page through the calendar or try to remember which dates the tradeshow is). Then after the prospect selects the date and time, all follow-up emails and reminders are sent via Marketo for tracking.
This is a feature that would come in handy for me on a regular basis.