Excellent job!
And if want to now what to do...
1. in gfx/interface there are some most obvious files - you can alter them to be more modern, eg. changing dollars to coins. Also you can play with sidebar_header to fit hanging flags. Lots of minor interface things like btn_radio.bmp can be changed.
2. gfx/interface/button area contains of files from sidebar. Enough of that crappy paper next to the flag or that very medievalish btn_slider.bmp.
3. gfx/interface/eyr is next load of *btn* stuff and some backgrounds.
4. Some of gfx/interface/frontend you have already done. Maybe altering bg_start?
5. gfx/interface/menu is just changing files to more blue version ;-) I am sick of this gold thing
6. gfx/interface/peace resolution is used in peace resolution screen - how surprising - maybe some tanks, fire, etc.?
7. In gfx/interface/reorganisation you can change that cavalry to some tanks etc.
Long story in a short way - look through the files in gfx/interface and do some nice, completely new interface
I know it is labor-consuming, but no one tells you to do that tommorow...