Bringing in long haul aircraft is not the answer, AirAsia's long haul operation is marginal at best. The 777, especially the older birds are not exactly fuel efficient.
Keep it simple, low cost A320 flights under 5 hours, and keep that cost base under control. To be competitive with the likes of AirAsia they need a cost base competitive.
Tiger/AirAsia/Jetstar A320 operations are good examples of the basic low cost model. Thai Smile still does not scream out "LCC" to me.