Ishimael made reference to the fact that he wispered in Artur Hawkwings ear and sent his son and his armies across the seas, and in doing so he secured two fates, one that has come to pass, and one that is yet to come. (paraphrased)

 

now I presume the first fate is the destruction of Artur Hawkwings Empire, what is the second fate?

I had thought it had to do with the corrupt form of the Prophecies they took with them, so that somehow that will cause the Seanchan to secure the other fate.

Yeah, more specifically it could mean they won't ever ally with the Dragon because they expect him to submit to the Empress due to the prophecies, or it could even mean the destruction of the White Tower.. although the latter is less likely given he has rats in there already.

Their return?

The one yet to come could perhaps have been the fate of the White Tower? Only thing I can think of.