Although there is a possibility that Ishamael could be Moridin, I do not think so. I think Moridin is Beidomon, the Aes Sedai who helped Lanfear bore a hole in the pattern and allowed the dark one to escape. Both Lanfear and Moridin seem to have high standing with the dark one, and although Lanfear does not tap into the true source, Moridin certainly does. Who else to reward but one of those responsible for your freedom? And there is mysteriously no mention of what happened to Beidomon. If Lanfear survived, why not Beidomon?