GTA is innovative, but only to a certain degree. That's why it's so popular, very few innovation, mostly known material. If you stray too much from established concepts, you won't sell anything.
The problem with Tempest is the gameplay. It's basically a shoot'em'up (it was a 3d Space Invaders at first...) with some seriously unfair mechanics.
If anyone in here needs to refresh their memory on Tempest,
here is a pretty good clone with a few added features (just don't collect powerups if you want to play it like Tempest 1). Tempest is so awful, I bought it for DM5 and gave it away shortly thereafter. I hope Unity is better.
Mario: If so, can you name a few well selling abstract or heavily innovative games?