Add an alternative sub fee
take your gold and trade for gems, use the gems to buy more space. Now you have bought more space with gold. Solved.
The game was just not designed for a sub fee. I think it would take too many potentially disruptive changes at this point.
If you want to pay a monthly sub, just spend $15/month on Gems. Support the game through the method they have provided. No matter how you fund them the money will go to supporting and bettering the game. These improvements may even include better security from bots resulting in reduced or even removed restrictions someday. Adding a monthly subscription fee will only result in a division of the player population into the paying subscription “Entitled Elitists” and the non-subscription players.
So NO… DO NOT EVER add a subscription fee… I feel safe knowing ArenaNet will never do this to us anyway…