devtrackers.gg
Well, WAY back when battlefront was considered bad (this was before the Solo update and after the Last Jedi season) there was an AMA on the forums.
I, called HypergamerMk2 at the time, asked a few questions that the community wanted answers on, but then added "but I think I only speak for myself when I say, will Rex ever become a playable hero?" And u/F8RGE replied (to that question) "You're exactly the kind of men I need in the 501st.
Fast forward to the geonosis update. It was that someone at DICE said Dual wielding was being worked on (either that or it was datamined). We of course know now that as referring to the ARCS, but Rex wields two blasters as well.
More recently, Guillame (think that's how you spell it) tweeted that if they were going to add a Jetpack hero, someone else was in line before Jango. When he said that, everyone immediately thought Bane or Sabine, but Rex used a jetpack frequently in the Clone Wars, so it's possible for it to be referring to him.
There's also the fact that season 7 of TCW is coming out soon, Ben's tweet stating that they have more to do, and the evidence supporting Ahsokas addition, and who best to pair with Ahsoka than Rex?
That's all I can come up with. I'm sure there are more evidence out there, but I just can't find it.
I did not reply with that quote.