I don't really think that's a good idea to begin with. First of all, we all know this is just a hack n slash with grinding elements, putting subscriptions on it would just simply zap away mostly all the majority of the players that remained.

If you want the game to be economically viable and have player consider investing in it, they ought to come up with a new mechanic, something instead of entirely mouse clicking your way.

Putting up money toll walls won't again, cut it.