They seem to have an interest in lung diseases with a few private company acquisitions of late and though it wasn't their best deal Myogen a while back.
UTHR and Actelion always get mentioned a lot probably because they are mature and perhaps nearing peak sales (but I've thought that for a while).
It seems to me GILD likes to acquire earlier (I believe a few of the private companies were still P2 assets). Along those lines as a dark horse speculation I wonder if they would consider a company like FGEN which could bring them into a few bigger areas though not the near term revenue potential.