Vandal Savage: Definately one of the series big bads, he was the main villain of the S1 finale and had two S2 appearances. He'll almost definately show up once more.
Lex Luthor: Again, with several appearances in both seasons and a hint that he'll run for president pretty certain he poke his bald head in again.
Ras al Ghul: This one is pretty much hoping but he is one of the most asked for villians and as we saw in S2 the crew listends to us. Ras just has to show up.