Consider making something like smite did with its god pack that for 15-30$ you get all present and future monsters & characters.
Right now people that want to play this game single player mode to practice and learn different hunters without dragging their team down in the process cant accumulate anything, since 7 or so hours into it you start receiving a message that you will get nothing at all in training mode. So you will never unlock anyone or anything.
Your only option is to go multi whether you want to or not, or buy a founders key / game from a 3rd party. That is a lot of money that is going to third parties instead of to developing more content for the game.