Shujinko can't actually morph, that was just a gameplay gimmick to allow you to play as and learn the fighting styles of every character in the game while playing Konquest, because MKD Konquest was an upgraded version of MKDA Konquest, which was "training mode with a story".

The way his power actually works is he just copies people's powers. Sort of like Rogue or Mimic from the X-Men.

I think it's pretty obvious Johnny Cage is going to have a big role or be the next leader. Rayden's comments about him to Liu Kang was one sign as well as his ending with all that power that comes out when he's in Rage Mode.

I'm pretty sure Raiden would've said the exact same line about Johnny in the old timeline too. It was just to explain to Liu why he wants someone who acts like a jackass on his team. It was more a comment about personality than anything else, like "Yeah, he was a jerk to us 'cause he doesn't believe yet, but I can tell he's the sort of guy who will do the right thing."