A modern browser that can render all the web pages the N900 can't currently handle would be very useful to me.
As would a Nokia Maps replacement with the following features:
1.Renders vector maps live on the device (using OpenStreetMap data or something) and doesn't require either pre-downloading large amounts of map tiles or downloading such tiles on the fly over slow/expensive cellular data connections. Nokia Maps does this but its map data isn't exactly kept up-to-date.

2.Has a good database of locations/points of interest/businesses/landmarks/whatever and a good search for them (connecting to the internet to handle this database would be perfectly fine and in fact would allow the DB to be up-to-the-minute current). Much easier than having to search with Google to find the address then trying to use the clunky search interface in Nokia Maps to find the actual location and how to get there from where I am now.

3.Is able to use GTFS data (published by transit agencies) to render the locations of public transport stations/stops on the map and allow me to search for/locate a specific transit station/stop. GTFS-realtime support that can track and display the public transit vehicles near where I am looking or can follow along with whatever vehicle I am currently riding would be great as well.

The other thing I want is an upgrade for Fahrplan so it can use/display the mapping/routing data that its getting sent back in the JSON it gets gets back from the backend server for my transit agency (I was the one that added support for my agency to Fahrplan but I haven't been able to implement support for doing anything with the mapping data). More specifically something that can display the walking maps showing how to walk from where I am to the stop I need to be at, from one stop to another when transferring or from the final stop to my destination is what I would like.