Here are some ideas on how you can make additional cash that won't cause in-game imbalance:
1) "Minion of the Month" club - Charge $2/month and on the 1st of every month every subscribed and paying member gets a unique minion.
2) "Mount of the Month" club - Charge $5/month and on the 1st of every month every subscribed and paying member gets a unique mount to ride around on.
3) FC House Jukebox with additional/customizable soundtracks - Implement a jukebox for FC houses, and allow people to purchase additional/custom tunes for it.
4) Access to a special "VIP lounge" location for paying members only - Charge a small monthly fee, and let people who pay the fee enter. Maybe have the lounge sell a few unique vanity items/potions/minions/mounts. Nothing that is greater than the top gear in the game, but, stuff that is unique to VIP's.
I'm writing this to warn you SE... the path you're going down in a very slippery slope that sets an awful precedent.
While $2 a month (in perpetuity) is not a big deal for most of your customer base (myself included), you're going to come to other points where you can either nickel and dime your customers, or not release promised content. If you continue to nickel and dime us, you're eventually going to reach a tipping point where a sizeable percentage of us (myself included) are going to leave.
If you really want/need to go into fundraising mode, stick to selling thing that don't give people who pay for the additional fees distinct in-game advantages (having 20 more spots on the market board and additonal venturing are big advantages) over those who do choose to pick the fees. An example of this would be the Fantasia potion. No one has a problem with you selling those. Selling market retainers is a completely different ballgame. Tread lightly.