Calculate priority based on date.
A function to let have newer posts higher priority, would be nice. :-)
How do you want to calculate this? Should the last x days have more priority?
I like that idea of x days have more priority. I do think you should also consider allowing more priority to specific pages as requested below.
...perhaps this, and perhaps based on how often or when a page was last updated. For example, if the page is 365 days old its priority is 0.5. Is it 60 days young or was it updated within the last 60 days, its priority is 0.6.