I'm a fan of the Rome Total War series and my idea ( I'm sure some one will pick this up because it's such a great one) is to create a strategy game involving the Gaelic tribes. Specifically the first objective would be to take a small tribe led by a leader with interest in uniting the tribes of Gaul. He would start out with some basic militia kind of units and try and conquer as many other tribes as possible.
After a certain period of time he may become powerful enough to ally himself with the Germanic tribes or other strong Gaelic tribes, and also Carthaginian forces coming from the South. Of course, Julius Caesar will arrive at a certain time ready or not and begin conquering everything in his path. By that time hopefully some of the Gaelic tribes are allied and developing technology and discipline, which in game terms would mean new units.
He wins if he can fight Julius Caesar and the other hostile tribes around him to a standstill thus gaining independence for his young nation.
Anyways, just a thought.