Hmm, let me think:
Firstly, you may need to get some H3 graphics to play with. Extract your .lod files from Data folder using resedit or Deftool.
Config\settings.txt may give you some insight of what particular graphics are.
Some info has been posted here. Also, Heroes Community is a great (greatest) source of information.
Ok, now, what’s here to do?
We don’t have really any “sketeches”, as if we had, we would simply put the layout into code. New interface just needs to be designed, possibly making it as close to original as possible.
An urgent issue is a support for 800x480 resolution. Many windows need to be tweaked to fit it:
- Main menu
- Map browser
- The combat screen (probably with help of edge scrolling)
- Castle screen, fort view screen
- Hero screen, exchange screen
- Thieves’ guild. Keep in mind request to add more player colors in the future.
Higher resolutions are similiar, it’s about main menu and scenario selection screen. We need huge (1920x1200) picture that could be cut or scaled down to fit all the resolutions.
What could be improved?
As TODO list shows, ideas are as follows:
- Advanced option dialog, allowing us to choose resolutions and color depth.
- Quest journal with minimap, similiar to Heroes V preferably.
- New stack experience dialog - It’s just not good enough. Since it’s not a part of original game, feel free to do anything with it. Also, additional button to returning stack artifact back to hero would be helpful. Think about adding some scrollbar for huge number of bonuses.
- Creature info, so it can hold up to 10 digits and info about ALL abilities and active effects, as in H5. probably done by gradual expanding of the window.
- Mithril button for adventure map to make use free space.
- Commander button on hero screen
- Wogification button in scenario selection needs to be replaced with “mods” or “mod browser”.
That’s all I can think of for now, I bet it’s already too much 
