Just a couple of random thoughts from my Ham background:
1. Support for shapefiles. On top of the shapefiles discussed elsewhere for road support, I have a collection of Maidenhead grid square overlays in shapefile format that I overlay in Xastir. Makes it nice to use an unambiguous grid system with other Hams, assuming they know what a MH grid square is. I'm partial to Kenwood's and their D700 calculates the GS and sticks it on the display if you have a GPS connected.
2. Support for APRS connections on a TNC so I can feed heard positions on the air into the display. Plotting yourself (from the GPS) plus who's around you on APRS (heard OTA, if they're squawking) would be cool for situational awareness (who's around me and where are they, without tying up the air).
3. Support for APRS via the APRS-IS over a mobile broadband connection. If you wanted to get really sexy, you can get your own position from your connected GPS, figure out what the radius of the display is based on display zoom level, then submit a filter string to the APRS-IS connection to only send you the objects within that radius. Bonus points for filtering on object type, so I could say show me everything, only WX stations, etc. By letting the server do the work, you save bytes (good if you're not on Alltel) and cut down on the processing (tie the filter to what the display is zooming at and everything you get SHOULD be relavent).
This is quickly turning in to a GRLevelX, Xastir, <insert your own favorite here> mashup, but could be a very useful tactical display.
We run a local weather net of Davis stations squawking APRS on an alternate frequency here in the county (I gate it to the Internet for the local EMA). Are they perfect, no. Do we get a good idea when the front passed and it gets really windy on the west side of the county? Yes. Not perfect, but useful to know, and our FD spotters don't have to call in "it's windy here" without defining windy.
If I could mash that into GR2AE, I'd never leave the computer
EKG