Make sure you work out relationships with beta testers so that you can do a lot of onboard "tests". :-) That was probably the most fun part of the job (other than reverse-engineering the GPS models on the market at the time).
Beta testing is part of our DNA... there was a group of boaters using this app for months before it shipped, in many different cruising and sailing scenarios.
We shipped the first version to beta before we had built many of the features. We also get some broader test data from beta testing Gaia 8, though that doesn't help with design so much as QA.
I saw Nobeltec just released an iPad app where they drape NOAA raster charts over DEM data to get a 3D effect. I think we could whip up that feature pretty quick, but do people really use that a lot?
The 3d rendering in the original PC product was kind of a "why not" with OpenGL acceleration just becoming somewhat common. It was very cool looking and was a good combination of bathymetric data with the charts of the time.
Mind you, we still had to deal with different geodetic data depending on the chart supplier. Seamlessly navigating from a NAD27 chart to a WGS84 chart was probably the most important feature at the time. Interfacing with as many random pseudo-compliant NMEA 0183 devices was the other. Autopilots that don't respect zero padding is a fun one when it comes to headings. :-)