This is a game idea I've had for a few years.

Farmer Joe needs to get his animals from one place to another. The animals walk along the farm roads until reaching an open gate through which they will go. Farmer Joe has a number of items to help him herd the animals. His trusty dog, his old tractor and of course himself.

Sounds nice and simple but, goats will jump onto and over the tractor, pigs ignore the dog etc. By creating a map of the farm and using gates, the tractor, dog etc the player must herd the animals from place to place.

Animals I had thought of
Sheep : dumb animals that do exactly what is expected.
Cows : Similar to sheep but maybe they ignore farmer Joes son.
Bull : only affected by the tractor and will ignore Joe, dog etc
Goats : ignore the tractor
Pigs : Ignore the dog

The roads of the farm have many gates. Gates can be opened or closed by farmer joe or his son. Animals will not walk through gates so the player can open and close the gates to affect the directoions the animals can walk. Using the various tools at the players disposal (Farmer Joe, the son, the dog, and the tractor) the player can also modify the paths availabel to the animals. Obviously the tractor needs a driver before it can be moved.

Various levels can be created
Level 1. A simple farm, with one gate. The gate needs to be changed and the farmer needs to chase the animals
Level 2. As above with more gates
Level 3. As above but with a circular road requiring one of the gates to be changed after the animals have passed by.
Level 4. Add in the dog and road junctions without gates
Level 5. Add in the tractor (possibly removing the dog)
Level 6. As for level 2 but two sets of animals to move to two different locations
etc
etc
etc

T junctions are easy to use in a level. The gate would be hinged on one corner and would be able to close off two different routes.
A four way junction woudl be possible with two gates or with the aid of a tool.

Issues...
1. What happens if a herd of animals gets to a juntion witrh more than one exit. (I think depends on animals type - sheep/goats go only one way, Cows/Pigs split into two groups). Could this be used as a strategy to accelerate results?
2. What negative effect do various tools have. A noisy moving tractor may scare the cows, the barking dog may panic the sheep.
3. Scoring, possibly timed levels, possibly other goals.

Let me know what you think.