I have little to contribute to this discussion really my 2 cents: I believe dragons can be warged! Whether they will be or not remains to be seen.
I do however have a comment about this:
Unless he pulls a "Sword of Shanara" where a druid can pass all his knowledge to another druid, Bran will still be in training when the series end.
Bran is a powerful warg and greenseer. No "Sword of Shanara" is needed here. All Bran needs to do is relax enough to slide into the weirwoods, just as he does Summer and Hodor. When he does he will know everything the trees have ever seen, for as long as they have existed, period. Past, present, and maybe even future, because the acorn, the tree, and the stump all live within.