After another review of the episode, it seems that celestia and Luna have something else in store for twilight.
I think they want to make her an elemental pony like they are. "twilight sparkle"? kinda sounds mystical or starry. I dont know what kind of element she may be other than magic.
we have candence who is love and peace... then celestia of the sun, luna of the moon ... blue blood is just a conceded unicorn that probably has not reached that level of maturity yet,but very powerful in some way.
any thoughts?
Lessons on the proper use of dark magic from Luna. man that would be epic.
As for her element, I would say that Twilight is the time of day between day and night, Twilight Sparkle is and has been the one to unit her friends on more than one occasion. She reunited Celestia with her sister with the help of her friends who she also united into one cause. If I was to guess based on all of this I would say that she embodies the element of Unity.
I think she may simply become princess of twilight, the state between night and day. I believe that the test was to see if she was princess material.
As for Cadence, I don't think she's a part of the times of day alicorns, nor of love and peace - I think she is simply the Crystal Princess, as the Crystal Ponies called her. Notice that her cutie mark matches the Crystal Heart and was glowing with it. It was at that point that the Crystal Ponies yelled, 'The Crystal Princess! She has returned!'
I think all these facts prove that Twilight is the missing princess, the one to control the times where Luna and Celestia have no purpose.