Multiplayer asteroids (with bots if too few humans are playing). Asteroids are removed. Finite, replenishible energy for the projectiles and a shield (running off the same energy) are added.
I'd need to spend a fair bit more time to develop the skill to beat the medium-difficulty-CPU consistently but I don't know if the game would be worth the time investment.
After a while, I started to realise that the rules might actually encourage some strategies. Running into an enemy with the shield up is an easy-but-risky way to kill, offset by the fact that you bounce back when shot at even if your shield is up. When firing, you need to make all your shots count, trying to predict your enemy's future position and firing enough that they can't all be absorbed.
Bullets were hard to see (on a CRT), energy bars impossible to find.
Twice, the enemy exploded in a 1v1 and the score didn't change. Just now, I realise this may be because it killed itself. If so, having its own score reduce by 1 (even going negative) would seem fairer.
I think I'll probably play this longer at some point in the future.