I think Aloha would be a much more likely acquisition target for Southwest than Hawaiian, but I still wouldn't hold my breath.
The idea itself actually makes a lot of sense in some ways. The fleets match up perfectly, and over the past few years AQ has linked its interisland network nicely with several WN cities on the mainland (BUR, OAK, RNO, SNA, and LAS). Both the mainland-to-Hawaii and interisland markets are dominated by leisure traffic, and thus are very well suited to WN's cost structure. Furthermore, there has been discussion in the past about whether WN, which has been steadily adding longer and longer flights to its network, might consider adding service Hawaii. This would be a pretty quick way to do that.
However, there are also plenty of obstacles. As was mentioned, WN does not like (or need) to go through the trauma associated with acquiring another airline. Also, operating 737s to Hawaii poses operational issues. I'm not sure if WN's fleet is even overwater-equipped, let alone ready for ETOPS (although I'm sure there are others here who know more about that). It's true that AQ is already certified, but that might require operating the transpacific and interisland flights as a separate division, which also goes against the WN philosophy of simplicity and fleet-wide standardization.
So, while the idea is not entirely illogical, and certainly makes more sense than some of the oddball rumors we have heard on this and other message boards (HP buying 747s again, AS flying to London, and so on), I still don't see it happening anytime soon. But in this industry, who knows...