Sakurai made it explicitly clear that Rex's costume was added because he missed the mark for both the base and DLC part of the game. There would be no reason to make his costume and apologize for his absence just to make him one of the DLC characters anyway.
On the other hand, it's believable that Rex could break that trend should there be a Wave 2 Fighter's Pack. By that time, Sakurai might give Rex a second chance and make him playable. We'll just have to wait and see. It's now or never, honestly.
It'd be nice if there's a Wave 2, but I'm not going to count on it. So unless they penciled in a vauge 'Xeno 2 character' when they were deciding DLC in advance, I don't think we'll see Rex unless there's a Wave 2 or version 5.5 or something.
Screw Rex, why not just put Homura and Hikari in Smash? Have Homura as the main character and tapping her portrait in the character select screen cycles between her, Hikari and the fused form. I'd buy that DLC.
Screw Rex, why not just put Homura and Hikari in Smash? Have Homura as the main character and tapping her portrait in the character select screen cycles between her, Hikari and the fused form. I'd buy that DLC.
I'd be real surprised if one of those two didn't end up a DLC fighter as in terms of popularity they do overshadow Rex.