I dislike the (insert character name here) secret Targ threads. Aegon is already knocking about in the story and whether he really is Rhaegar's son or a Blackfyre - that is one Targ reveal already. (Because what are Blackfyres really but Targs with a different name?)
I do believe that Jon is Rhaegar's son with Lyanna. I cannot think of anything else that Ned could have promised his dying sister that would haunt him like that. Ned always carefully refers to Jon as his blood - not his son.
Violence towards children is a big no-no for Ned. 1. He quarrels violently with Robert over the deaths of Rhaenys and Aegon. 2. He allows Cersei time to get Joffrey, Tommen and Myrcella out of town. I think he would lie to protect his sister's son.
Making any of Tywin's children Targs robs their story of most of its intensity. Reading about the Lannister's glorious dysfunction is one of the great things about ASOIAF. Tyrion is the most like Tywin, and they are both too blind to see it. Both are brilliant and ruthless.
I know that a lot of fans say that Jon's rise to power was too easy, but it is not like the rise to power has made his life better and cushier. Some readers act like being Jon makes your life a bed of blue roses. But, the dude is freezing his backside off and surrounded by mutineers. Not to mention hordes of the undead are coming led by their otherworldly masters. Yeah, Jon's life is so sweet.