I think that's something that will be in their hands. They were in love with each other once. With the cases appearing and Lin growing closer with Tenzin's family, I'd say it might be a natural occurrence if they cross the line. Given the standing of the two, I'd bet on Tenzin initiating any sort of contact beyond their friendship. Whether unknowingly or knowingly, he may either confess to Lin or just initiate some form of affectionate display - a kiss, a hand brushing her cheek, a sincere "I love you" before bed, etc.