I like the idea of a Level Editor/Creator. If I may be so bold as to expand upon it:

Be able to place buildings, terrain and castles.
Set objectives as already seen in the game and maybe more such as mobile bridge deploying. Maybe even multi-stage missions to be done with one machine.
Add enemies, sheep, villagers to, um, relocate...
Terrain and limitations for the machines location (so it must be in an area or inversely can not enter an area). Maybe use terrain features to enforce?

Above all the ability to save and share these user created levels would be great, really giving the game a long life span.