Add a game mode, where players can customize most, if not all settings of a game - such as the biome, forest mysteries, additional modifiers (from map tiles), available races, resource density and so on, so they can train or practice strategies. I suppose it should be separate from the main campaign.
I don't know how exactly the map generation works, but I think it would be great if we had the ability to access the seed of the map (if there is any), so we could share the exact game conditions with friends.
Maybe make players pay meta resources to activate modifiers (exactly the opposite from the way map tiles work).
Modifier unlocks could be tied to their respective deeds - for example: In order to unlock the ability to select "shattered obelisk" modifier, you would have to have beaten game with "shattered obelisk" modifier on X difficulty or harder.
I think it would be great to play a game with all (almost all) modifiers and forest mysteries active at once.
I agree. It should be an extra sandbox mode but, at the same time, it could be integrated in the campaign as a type of very rare biome or event (question mark) in the campaing map where you can build with or without time limits and choosing all setting of the game (as suggested).
Additionally, in that mode, being able to restrict the number of available blueprint and cornerstones would be great !