Your new mane character idea intrigued me because it's what other I talked to have thought of. Many have said that it may seem unnecessary some like this idea. Then there are those that say it's unlikely, but I see this as a possibilty. Thought the more I think about it, it may be forced instead of put in for the show.
Hasbro is in it to make money and so far this series had made them millions, they are going to want to keep I going till the last drop. Anyways making a new character to be in the mane six has its business advantages. You can sell new toys, and people with the mane six may want to add this character to complete there set.
The addition of a new character may extend the life of this show as well, which will help them out.
Now I'm not saying this is whats going on in there heads, but you never know.
Besides even if a new character is forced apoun it is likly that the writers will be able to make this new character work.
Anyways the point in tryin to make that it isn't as far fetched of an idea as it seems, but we also don't know if it will happen.
Only time will tell.