Jacqui. There isn't much point in introducing a next generation if the old one is going to always be there as well. Not that I dislike Jax but IMO Jax, Johnny, Sonya, Jacqui and Cassie don't all need to be together in every game from now on.
Johnny, Sonya, and Jax used to miss games back in the day and they were still fine games, so I think letting Cass and Jacqui take over for at least one game would be fine. MK2 had no playable Sonya (or Kano) and is remembered fondly. MK3/UMK3 had no Johnny and is also remembered fondly by much of the community.
Again, not saying they need to go away forever. I would hate that since I like all of these characters but they can sit one out every once in a while.