I'ts not easy to really predict what they are going to do with the show. Its been tried before and usually isn't fruitful. But anyway, this is something I've thought about quite a few times before. There are a few problems with it:
Not all of the main six have either a horn/wings
those that have wings already will require horns, which is a pretty big change as far as actual looks go
those that have neither will have to acquire both, which will effectively change their character quite drastically.
In order for this change to happen, Celestia has to make it happen. While you know she has a strong connection to Twilight, the same isn't exactly true for the rest of the mane six, whom she knows, but not on a personal level.
Adding to the above, they would have to prove themselves to celestia, meaning she would have to assign each of them a task they have to complete on their own.
Celestia doesn't yet have any incentive to make them Alicorns.
On that last note, you have to kind of wonder why Celestia made Twilight an alicorn. It wasn't just a kind gesture, she has a purpose behind it.
My theory is that Alicorns are not immortal, and Celestia can feel her time coming to an end. As no other Alicorns exist that fit the bill of leader, she has to "create" her own. So this means either the very end of MLP will show celestia dying or something similar and Twilight taking over, or, a more reasonable prediction, the Mane6 take over, which does not necessarily mean they all have to be alicorns.
There is a lot to consider here, and you have to remember that Twilight always stood out from the rest of the main six. She wasn't like them, she is better, you can't really argue it.