Shadowfate

Personally I believe that MLP is heading towards having all of the mane six become Alicorns and replace Celestia and Luna. It does make sense if you look at the thing as a whole. (about to dive into deep head-cannon and theories) Princess Celestia knew that since she no longer controlled the Elements of Harmony that she needed to find ponies who could wield them. So she began searching for the ponies destined to save all of Equestria. She set up the school for gifted Unicorns for the sole purpose of finding Twilight. She began training Twilight herself once she found her. I would also be willing to bet that Celestia had found the rest of the mane six by observing them from a far (what do you think that telescope was for?). She arranged for those who already didn't live in ponyville to somehow make a living there. Rainbow Dash and Fluttershy both hale from Cloudsdale, and somehow ended up in good ole' ponyville. Pinkie Pie also was raise outside of ponyville, but somehow ended up there. On the 1000th year after banishing Luna to the moon Celestia arranged to have the Summer Sun Celebration in Ponyville and sent Twilight to check on preparations. It was already arranged by Celestia for Twilight to meet up with each of the other elements who were all in charge of one part of the preperation or another, with the exception of Pinkie Pie, whom Celestia trusted to find Twilight without any help. Celestia would have been counting on Twilight to discover for herself that her new friends were in fact the Elements of Harmony. Why else would she have left the task of defeating Nightmare Moon to Twilight? Her famous line directly after MMN's defeat was "I knew you could do it" That just screams of Trollestia's devine planning! Now that she's found new bearers of the Elements, she won't want to let them die and leave Equestria without it's best form of defense. She needs all six of them to become alicorns so they'll always be around to save Equestria. Besides, She's much older than a 1000 years old, I'm sure both she and Luna would be looking forward to the eternal rest of death, knowing that their beloved kingdom is safe forevermore. That would leave Equestria to be ruled by a total of 7 princesses in the end, unless Cadence gives up her Alicornhood so she won't outlive her husband and any possible children. Which I know that is whole theory is pretty far-fetched, but if handled properly, I'm sure the team could pull it off, and they could still keep the show running even with all the mane cast as princesses, by having the CMC take over as the mane characters of the show, possibly as adults and follow their adventures.