Literally all of what you said has occurred to me...except for the stuff about Zuko. I'm not familiar with the Last Airbender.
The fact Kylo is established as someone who isn't scarred and damaged like Vader was (and we see him without his helmet at least twice), I've even wondered whether he'll be redeemed fairly early on in the trilogy, and switch allegiance. Maybe too much of a stretch, seeing as we already have Finn doing that.
Not that his face would have to be pretty and unspoilt to become an enduring good character, but it works better thematically. For instance, had the OT been written differently and carried on, I can't imagine Anakin hanging around looking like the spawn of Freddy Krueger.
Family, reconciliation and redemption have always been strong themes that have gone hand in hand in Star Wars. I would not be surprised if they wanted to keep that going in this new trilogy.
I would imagine that if thats want they're planning to do with Kylo Ren, the change would most likely be gradual and will encompass the whole trilogy, not unlike Vader in the OT, another Skywalker family squabble with the rest of the galaxy in the middle, things tend to get a little messy when that happens haha glad i'm not the only one that picked up on this scenario as a real possibility.
If Kylo Ren and Rey are siblings, it kind of makes you wonder why she's just trying to mind her own business and hide away on Jakku. Something from the poster gives me the feeling that she has been trained as a Jedi but for whatever reason she's turned her back on being a Jedi and sought a life of solitude instead, hence why she uses a staff rather than a lightsaber. What happens in this first film will most likely force her to face that part of herself in order to protect Finn that appears to be untrained with a lightsaber and pretty terrified when Kylo Ren comes for him in the trailer after his initial show of bravado.
Kylo Ren perhaps was not trained as a Jedi because his mentor/father (Luke perhaps?) could sense something potentially dangerous in him and remembering what Obi Wan told him about his failure in training Anakin, decided to try to avoid making the same mistake, Kylo Ren gets training from the Knights of Ren instead and now has an axe to grind against his old mentor and all Jedis in general. All speculation of course but its fun to run with it. For the record, I think it's Han Solo that Rey is crying over there, will be interesting to see what's actually happened there when the film comes out, see whos right with all the jacket debate going on right now haha