I think that playing the roster unbiased and seeing who you have the most fun with is the best. In Marvel vs Capcom I went all in on a boy I have supported and dreamed of entering that series for most of my life...Iron Fist. Words cannot express how bad it felt when I got a character with zero tools to survive the environment he was put into. Combos that couldn't start because his overhead forced them into a bounce that removed his crumple and thus all his damage. No tools in the air including an air throw he couldn't convert at most heights. His best tools locked behind a prerequisite of doing 3 rekkas first and his SF3 counter being relegated to a shoryu motion on his chi powers. Damage, pressure, incredible wave dash...none of it useable because he couldn't move.
In Injustice I went in hard on Green Arrow. Hell, I fulfilled a promise and dressed as him and danced the Bernie when he was announced and modded his boards initially. But he was a pressure character with T-Rex arms, no damage, and was entirely reliant on resets as he was an archer who couldn't outzone anyone. He wasn't bad , but he was quickly irrelevant except in the hands of incredible footsie players who can make a living better IMO by switching to Batman.
Point I am trying to make is...the most fun I had in Marvel was as Chun or Sentinel. The most fun I had in IGAU was with Bane, Hawkgirl and Shazam. We dont know who are mains are by the character history. We know who are mains are by playing the game and finding who we really make work and connect with during the rhythm of a match.
In MK10 I am going in and playing. I hardly got to experience MK9 because the few months I was in were right before IGAU so I didn't hang out long. I mained Smoke then and played a lot of Sonya/Kung Lao with that and enjoyed Ermac and Noob too...this tells me in MK10 I should just go in with eyes open and experience it before choosing a main.
Pick your mains based on who you have the most fun playing. Wait for MK10 and play it and choose after you get your hands on it. I'll be eager to see how my old favorites played, but I will be going in with intention to actually experience the whole roster and see who feels right for me. I feel everyone should do this.