I don't know if you are referring to Bangkok or Pattaya. In Pattaya, I feel there is a REAL need for an upscale pet shop. There are lots of "down market" ones, and I hate buying food there, cause you don't know how old it is, it's not air conditioned, and they don't always have a wide selection. There is also a need in Pattaya for an upscale dog/cat groomer. There are lots of down market ones, again, but I've yet to find one that really looked like you didn't have to worry or supervise too much. I would think a pet shop, with a groomer, upscale, marketed to farangs might do well, that had easy access with easy parking. I think there are lots of people with pets here, most seem to take very good care. You don't often see them, as they are usually in air conditioning inside.
I'm quite certain in Bangkok something like this, upscale, already exists, but my HUNCH is that there are enough people to support this. Remember, you are competing mostly against pet departments in Carrefour, Tesco and BIG C., but I find their selections to be NOT that great. There is a lack of big dog toys for big dogs, big dog bones, dried pig ears, etc., and especially sold in packages of more than one item.