Allow using WFS data sources for map generation.
Properties file would have possibility to list WFS urls, and Osm2GpsMid would query the specified WFS servers and combine the normal map data with data from the WFS server(s), so on the mobile device data from WFS is stored locally.
Somehow extend the style machinery to handle the layour for data from the WFS sources.
Consider also licensing - a map data license may not allow distributing a combined database of e.g. OSM data and differently-licensed WFS data sources, so either
1) inform Osm2GpsMid user about possible license conflict with distributing the map (I don't think there are problems using the map locally if not distributed)
2) have Osm2GpsMid create separate databases, one for OSM data, and one or more for other data