Rather than wait for suggestions to roll in here, just check in to Highrise Forum and note the ten most asked for features (which you may already have) eg:

Calendar/iCal link Easy multiple/mass contact processing, tagging etc SMS reminders Blackberry and/or Mac address book sync Categorise/filter tasks Simple Google or Atomz style searching

Calendar/iCal have now been added, task categorisation has been there from the start.

Thanks

Jake