Nothing! I hope this kind of advice doesn't feel odd or with ill intent, I really want to be able to play this game and I think the majority of people who can't afford to pay the third year pack, would pay for the game only!
If you add that option, don't forget to publish it in all your social apps, so of there were people who didn't buy it before, could afford it now
Again, I hope you don't take it in the wrong way, if so apologies!