The problem with mechanics based battles is that they're all routine. Look up a perfect strategy online on day 2 and prepare for farming.

A solo arena isn't a bad idea, might even be implemented in Gold Saucer. But the trap remains - if it's made by SE, it's scripted, therefore easily, easily farmable when you don't have to rely on picking up other peoples' slack. There will be no challenge once the videos go up.

Randomizing content might be better in the long run eventually, such as random battles in the arena, using random movesets in random environments utilizing random mechanics. That's just me, though. It's one reason I decided to try out Diablo 3 again...