They need to add more MGP rewards or more titles related to GS.

[Gallus]

unless you want to have everyone trying to get the high priced MGP items and then sell it back in the market with a ridiculous mark up or have RMT get involved somehow.

There is a lot of expensive stuff gated behind specific things (unhidden maps have zillions of glamour pieces, materias V = just bubblypots) so I can't see why this would be different.
Also, the RMT excuse is getting old. Everybody knows there is a huge amount of people in this game that uses bots, so who cares anyway. As for the "just playing the minigames for the sake of playing them", trust me that gets old fast, you need something behind it, I don't care if it's in the form of small amounts of gil, titles, glamour items or mounts, but there must be something more than what there is now or once people get all the stuff in there they basically have no reason to keep playing the minigames and GS goes graveyard saucer automatically (if they don't get old after 100 tries, they'll get old after 1000, it's just a matter of time if you don't add some sort of long term goals there, for some people it'll be the possiblity to make gil, for others it will be titles, but please nothing related to stupid cactpots, that's NOT FUN, it's a freaking lottery).