A few GUI gripes

[Edi]

Sorry, the UI is not moddable in the way you would like.

Nation flags can be changed as a feature of nation modding, but the on-map castle and temple graphics can't. What you can do is use the 1-9 keys to set the flags you want, including armies, castles etc, or you can pop them off for a moment and then back on if you like.

The only UI mods I know of are here and that changes the appearance of the backgrounds and the boxes, not the stuff displayed on the map.

As far as getting into modding, looking at the modding manual in the Dominions 3\doc folder is a good (and required) start.

For some of the most famous mods, look at this thread. In addition to the Conceptual Balance Mod (CBM) versions, it lists some of the best work of the community, (Expanded Nations being a compilation of many mods, WarhammerWorld another). Look to the Scenarios, Maps & Mods subforum for more lists and stuff.