I had a bit of a fridge brilliance (no, I'm not going to link to Tvtropes, it ruins lives) moment yesterday thinking about the whole Luna imprisoned in the moon business.
We know that Celestia imprisoned Luna/Night Mare Moon in the moon 1000 years ago, using the elements of harmony. We also know that Twilight and co. harnessed the elements to do something markedly different in the pilot episode(s). Finally, we know that it was not Celestia that released Night Mare Moon, the stars did (who controls those, if anyone, is up in the air at this point).
What I propose is that Celestia's imprisonment of Luna/Night Mare Moon was not the plan she had in mind. Celestia expected to be able to use the elements to do what Twilight and co. did, but failed for some reason. Possibly, the spark (friendship) that Twilight needed was absent when used by only one pony. Celestia, however could wing it, using her god-tier powers, and ended up imprisoning her own sister in the moon as an unfortunate improvisation. Moreover, she was unable to get her back.
This would explain why she hadn't put together a six-pony team to save Luna earlier, as she had to wait a 1000 years for the stars to free Night Mare Moon. It also explains her sadness over having imprisoned her sister for such a long time, she never meant to. Finally, it explains why she had to pull the batman gambit of sending Twilight to make friends and defeat Night Mare Moon, she was unable to use the elements to their fullest by herself.
Is this plausible, or am I just rambling here?