Other options besides AirAsia for BKK-HKT are Orient Thai and also NokAir flies DMK-HKT. DMK being the old Bangkok airport Don Mueng, much closer downtown. I've flown NokAir and it was cheap and pleasant. Very old 737 though, actually had the ashtrays ductaped shut haha.

If you take the Delta flight NRT-BKK, and might as well fly NokAir from DMK so you don't have to go all the way out to BKK again if you'll be staying in Bangkok.

Other option is flying into SIN as I also did. Tiger Airways flies SIN-HKT and I think AirAsia too. Likewise, the Delta NRT-SIN flight gets into SIN quite late.