I was about to post an observation that there are already WAY TOO MANY placenames - look at this screenshot for example. However, i suspect it may not be quite that simple, different countries will have different densities of towns, even small towns may be significant waypoints in very unpopulated areas, etc, and the map database that the maps are constructed from may be inconsistent from one country to another in terms of classifying towns/villages etc

Isn't this a layering or categories question?

I'd like to have more such indicators as well. Names or symbols, such as "castle", "antenna" and the like.

But since a tablet has only limited space, it should be possible to filter these objects, depending on zoom level or so. Consider

• traffic (highways, roads, railways, street-categories)
• geography and/or surface (lakes, rivers, caves, forest coverage)
• population (cities by size/number of residents, administrative importance)
• places of interest (castles, tourist attractions, landmarks of various kinds)

and so on. A look at the current consumer-GPSes might help. My garmin eTrex and other models have quite an elaborate categorization of such objects (though Garmin did, as often, not make much of it). OSM is a great starting point as well.