I'm going to pick whoever I feel most comfortable playing, and have most fun with. Preferably someone who isn't too rushdown-orientated or requires heavy execution, and either has good zoning or escape options (or both, ala Freddy). It's how I chose Smoke. If I went by who my favorite character in MK9 was, or who I thought was the coolest in the roster, I'd be a Scorpion main. lol.
That said, the first ones I'm going to try out in practice mode are Aquaman, Shazam, Deathstroke, Cyborg, Raven, and Ares. No particular order.
I am going to start with Batman (loyalty), Green Arrow, Shazam, and Raven or Killer Frost. I am a little worried hearing that Shazam is a grappler so I may mess with Flash or Sinestro if I don't end up liking him right away. I am going to be putting a lot of time into Batman though so I know he will at worst be a co-main.