Milestone 1: Render the main screen. Bridge. "SDL2" initially.
Milestone 2: Create the game model. (because otherwise boredom would slow down the project)
Milestone 3: View 1 - UI.
Milestone 4: Controller Main - UI.
Milestone 5: View 2 - map.
Milestone 6: Sound.
Milestone 7: View 3 - battle.
Milestone 8: AI.
Milestone 9: Hi-scores.
Milestone 10: AI scripting.
Milestone 11: View 4 - campaigns
Milestone 12: https://heroes3wog.net/did-you-know-second-edition/
Milestone 13: Multi-player.
Milestone 14: Plug-ins. (sound, resource manager, renderer, AI)
Milestone 15: Mods. (should take 2 minutes if all of the above are designed correctly)
Milestone 16: New things. (see "Whats the new?" at the file "project")
Milestone 17: Map editor.
Versioning: 0.* - alpha releases; 1.0 - proof of concept; 1.1* - to be decided.
The above could be wrong, could be right. I have no idea. Only one way to find out.