Here's a few to get started (I've just really seriously started using Calendar so I'll add more as they crop up.
1. Improve recurring events so an event can occur on more than one day repeated weekly. here is a screenshot of how Google calendar handles this.
Obviously this is a lot more flexible than the current setup as currently I need to add two seperate events when really it is just one. Also the two events sometimes show in the upcoming calendar events right next to each other as shown, which can be a little confusing.
2. Support for extra fields, certain things occur frequently like "Location" and "Entry Price" this would keep a consistent formatting. Currently it's a bit mish mash with different people interpreting the description tag differently. Even better if the calendar could be searched on these fields but that may be a bit overkill.
3. A better way of switching between weekly and monthly views possibly add a daily view as well. At the moment if you hit "view current week" when you get to the weekly summary there is no easy way to return to the monthly view.
4. Ability for members to watch events similar to topic subscriptions. This would alert members to any changes and send them a reminder closer to the time.
5. Ability to invite members to an event you have submitted and list those members that have confirmed, declined or are still awaiting a reply. Facebook-esque!
With those things Calendar would be a formiddable app in it's own right and would integrate into the community package a lot better, currently I feel it is useful but underused as their is not enough integration with the rest of the products.
I'm not sure if this final suggestion would work as it may be a bit of a mind leap for the average casual user but linking galleries to past calendar events could also be a nice feature
Thanks for listening :)