Forgive me if this has been brought up, but has there ever been talks of releasing the mountain that rides unit in a hero box or in some form other than the kick starter special? I have brienne and bronn on the painting table from the hero box 2 and they are fantastic, Im so glad to have these characters and it has made me wonder if we'll get the MTR. If im not mistaken he is a 4p unit which is really powerful and gives the lannisters something special in terms of utility. The tully unit (black fish i think) would be a nice box.
Short answer: we don't know
Fun answer:
It'll most definitely be released but there's not been any clues as to when and if the stats will remain the same. There has not been any mounted characters/attachments released so far so i believe there's a hint to be taken there:
The Targaryens starter set will contain mounted commanders and attachments. The Night's Watch Heroes Set #2 will contain Cold Hands (with Gilly and Sam). I am speculating that Lannister Heroes 3 will have The Mountain that rides. Starks are awaiting for Brandyn Tully Outrider commander / attachment.
So there seems to be quite a bit of mounted things upcoming. So my guess: during 2020
P.S. I've used him quite often and yeah, he is really strong. I wouldn't be surprised to see some slight changes down the road, but what makes him stand out is how unique he is in the Lannister faction.