Well, Gollum says he saw a man with four fingers, a direct reference to Sauron, so we can assume he has physical or at least visible form. I like to think he just kinda chills in the tower and uses the eye to watch over Mordor.
If he got the ring back I assume he would put it on his other hand, or another finger, until the ring gave him egnoughenough power to regenerate his missing finger. He would try to use the one ringOne Ring to control the bearers of the three elvenElven rings, Gandalf, Galadriel and Elrond, controlling any who were too weak to resist or remove the rings fast enough and perhaps turning them into wraiths. Either way with the one ringOne Ring he would eventually collect the other three, cuz he wants them for their powers and stuff.
As for his next step, I really have no idea. Perhaps burn everything? Perhaps bend manMan to his will? When he practically took over Numinore beNúmenor he created a Morgoth cult and killed people for giggles, till eruEru sank the place, so if I had to guess I'd say he would do the same, and then either the Valar or Eru would step in and kill everyone. Or, alternatively, he enslaves all races and launches a full scale assault on the Valar, and rescues his old boss from the Void. Together they destroy the Valar and finally reshape the earth as they see fit, with darkness and volcano'svolcanos and sicknastysick-nasty death metal album cover landscapes. Pretty rad. Unless you'reyour body isn't one that can handle lava and lack of air and the brutal awesomeness that is Morgoth's paradise. epic gutar rif and fade