This is a buy to play game, subscription is not the answer, it never was.
the only way to get around it is a cash shop, i havent seen any b2p MMO's without a cashshop. for those against it, you need to realize that having a game costs money.
the b2p mmos i can think off on the top of my head is, TSW, GW2 and ESO. all which have cashshops. which is mainly just cosmetics or cool companions. that is the way to go. no doubt.
any sort of limitation on the game for not paying more is an absolute nogo, things like exp boosts are discussable. but that i also think is fine, if people dont have as much time to level up then they can pay a bit of money to not fall completely behind.