If you look at the other airlines that fly the 738 ETOPS to Hawaii, you will see that WN would only fly from the west coast. I don't think they would venture too far inland. That said, I don't know what the official range is of the 738 ETOPS. If they ventured east, I would think Las Vegas would be their first choice.

As for interisland, I believe if WN starts flying to Hawaii, they would start an interisland service similar to what HA or Go! does eventually. I think it could be a money maker for WN and a benefit to the customer.