I remember seeing it when they announced Doman Mahjong, I wanted really bad.

I would like to see them make a set for purchase, especially since they've made Cactpot Party and proved the set possible to be made.
It's as you say though, There would have to be a sizable demand for it. (Although there are a LOT of players that like it)

That said, Cactpot Party is a brand new board game and does well. Mahjong is a worldwide game and would be considered a set for collectors and fans of the game and not necessarily FFXIV.