Technically speaking, to add system like Blitzball will require complete new set of UI, game system (using your existing character data), BGM, SE, and other features so it will cost a lot to develop such system BUT say if they can get it as a expansion system for fun which doesn't really affect too much of the main game play of XIV aside from getting trophies (for housing) and small items and stuff. It may take 1-2 years to even develop such system on top of current server.

How many would actually pay for it?

I for one would buy this type of expansion.

Pretty sure the Gold Saucer one YoshiP mentioned will be an expansion but guessing it wont have sport game like blitzball.

Also not sports but you know YoshiP might still be hiding the card game everyones been asking for so that's one mini-game already + gold saucer, that two lol