While nothing significant happens with the Seanchan invasion in the story we have, it feels like RJ left a lot of that story on the table intentionally.
With Mat's popularity to the reader, the potential for conflict between Randland and Seanchan, and some of the promises that were made to Rand by the Seanchan, I feel like an entire series or at least a small saga could've followed on if RJ felt like writing beyond the last WoT book.
It would take the series plot in a different direction while maintaining the WoT setting. RJ had always mentioned the possibility of him writing future outriggers based on Seanchan, I would presume in Randland and Seanchan-actual. Leaving so much Seanchan story left on the table would be a great way for him to lead into follow-on stories.