Regarding Lin, I still find it hard to believe that they'd be that subtle, though. In the end, this is still a children's show, which means that they're fairly open on the ultimate motives of their characters (I'm trying to think of an instance in which either AtLA or LoK hinted at someone's motivations/secretly involved and never outright said it). Even when they do less obvious things/more symbolic stuff, they tend to keep people's actions clear, even if their feelings/thoughts/internal crises are more left up to the viewer. And, on top of that, they sometimes still end up highlighting it--all of that subtle identity/Korra is more than just an Avatar/Korra as a person vs Korra as an Avatar stuff that was created through seasons one through two got blatantly stated in the finale.
tl;dr I want it to be true, but I don't think it is? XD
BUT COOLER MORE IMPORTANT STUFF:
So, Book Three: Change, has been announced. Spoilers for those who don't want to get spoiled: