Opening note: These are my opinions and advices about the income models of free to play vs monthly subscription in games in general and in istaria, based on observation.

There are many sites out there, that explain why monthly fee subscription model is on the way out, this isn't speculation, most of these show actual evidence and do surveys for this data. Also, take a look at some games that went f2p after failing with the monthly fee (star wars the old republic, aion, age of conan...so on). Even istaria failed with it once, before the current team took over.

In my opinion, fully free to play or buy to play is the way to go.

Free to play:
What shoud not be limited in f2p games:
-races
-territories/locations/quests
-instances
-crafting
-skills
-in-game items and equipment

What should be limited and/or put in as item mall items to maximise income:
-character slots
-bank expansions
-appearance items
-unique things like lairs/plots
-xp boosters
-in-game currency booster
-pets
-other items you can come up with but wont make the game pay to win.

How to further maximise income:
-add in the ability to trade premium currency for in-game currency, this will make it so people will buy premium currency to get in-game currency and vise-versa. Not only would people who dont buy prem currency be able to buy premium items, but also players who do buy premium currency to get in-game currency would provide the developers with extra income. This method also works as deterrent for currency slling sites and such.

Buy to play:
-same as above except people would receive a plot/lair and would have to pay an initial fee.
-paying for expansions.

My thoughts on istaria's method:
Istaria is still alive thanks to the developer's awesome skills, dedication and dragons, however, if a game that allowed you to be a dragon came out nowdays, and would have a f2p or a b2p model, this game would instantly die.

My suggestion: Switch over to a fully f2p or b2p model. Not only will it increase the playerbase, but would, in my opinion, provide the developers with a lot more funds to improve their game.

Thanks for reading.