A space strategy/tactics game with realistic physics. Basically ftl stops at the edge of a solar system, and then ships can reach a small percentage of c while traveling within the system. Lasers travel at c, but you have to anticipate where your enemy will be over long distances. Missiles or torpedoes are evn worse with the addition of worrying about fuel consumption to reach their target.

I think it would make for interesting dynamics. How would fighters/drone swarms work? Mines? Bases? How would ambushes work.