I say keep the roster small, about as big as it is in MK9. Too many characters usually results in many similar characters. I prefer quality over quantity. Around 25 is good enough. Even 20 is acceptable to me provided they are very unique.
As some others have said, MK needs a much more comprehensive tutorial, maybe character specific tutorial. Unlike pros, normal people have a hard time understanding a character. The ones that look good or are personal favorites often play in a way that is unattractive to the player. For example, Quan-Chi is one of my favorite MK characters and when I picked up MK9 I started with him. But once I dug a bit deeper I found out that I don't like the way Quan-Chi plays in this game at all - all that meter building and unblockable traps and everything doesn't suit my style. But it took me a lot of time to figure that out. I finally settled on Ermac. A character specific tutorial would have helped me not just to decide on a character very quickly but also learn it quickly. As it happened I had to resort to forums and youtube videos.
The Bosses of MK9 are very cheap. They basically couldn't manage to make characters powerful enough while obeying the game's rules. So they just broke them. While I'm not such a stiff person, at times it just feels plain wrong. For example, the bosses having incredible damage is logical as character's in MK9 have varying levels of damage. However, the bosses having more health is a cheap way of making them powerful because all playable characters have the same health. But even that I can live with. What I find extremely low is that even their normal attacks can out-prioritize almost anything from playable characters. Its almost like they ran out of ideas and brought out the white flag. They need better made bosses for the next game.
The AI needs improvement as well. With some characters its dismal. It would help with the bosses as well.
A complete overhaul of the roster doesn't seem like a good idea. I say bring back like 15 characters from MK9, bring back 6-7 characters from other MK games like MK4 or MKD. And finally add 3-4 ORIGINAL new characters. A non-creative character like Skarlet doesn't add anything.