Yeah I've seen the extra lines pop up since they
and for it being on half of his face, possibly him using half of his power, or the fact that he's half the personality - half of him fights to protect his friends yet the other half strives to fight for the sake of fighting and getting strong and as we've seen recently, he likes fighting more and the more he wants to fight the more lines he gets, probably due to him delving deeper into the hollowy goodness and getting stronger
but as everyone's seen - the opening of his mouth (that pic gave me goosebumps
I love it) and the fact that he's said 'maybe I'm becoming more hollow' means he's on the path the powahhh!