There was something a while back about Hasbro pushing the team to make a background pony a main character. When asked about the 6 stars at the con that shall not be named the writers said that must have been added by Jason Theeison in post production. While Twi becoming an alicorn possibly screws with the 2,2,2 balance (unless you follow the belief that Alicorns are simply a higher level of unicorn which Hearthswarming and Magic duel do kinda hint towards) bringing in a new character would only work if the character had retroactive appeal. For this reason I always felt the additional character, likely being a unicorn, would be Trixie. Outside of the Princesses and the CMC she has a really high following. While Derpy is a popular character they would likely avoid her to avoid further issues, besides the where's waldo scenes with her are quite fun.
Now as to this Sunset Shimmer character. The fact remains that the only thing the character has going for her as evidence is her mask. However, I would like to point out that as some others have stated she seems more akin to a villain/antagonist than a main character. For that reason I return back to my addition of Trixie as a 7th cast member.
She has retroactive episodes with several mentions in the eps she does not appear in. She also serves as a potential magic replacement if Twilight ascends for some reason beyond the elements (this I find highly dubious). Her episodes do cleanly represent a transition from antagonist to understandable and another ep or two could ease that into friend. For this reason I give my thoughts on her potential element, Forgiveness. Forgiveness isn't something that one acquires as easily as the other elements. You have to be with someone for a longer time for it to show.
Now while all this about Trixie is just spitballing theories I tie this back into the discussion so as to explain my tangent. If a recurring antagonist like Trixie becomes a 'good'/main character, you need to have a replacement rival in magic for Twilight. That is where I would see this Sunset character coming into the mix.
Also keep in mind that Season 4 has also likely already been written with them in the process of working on Season 5 now. If this character is going to be introduced in a movie or later season than Meghan didn't lie, but you also misinterpreted what she said. The question asked was 'Will she be in Season 4?' Meghan responded 'Nope'. This does not exclude possible later seasons or the rumored movie that has been floating around.