I've got a few custom suggestions that might be helpful for the website.
1: Username Styler - The ability to add custom colors + shadows under your name. This is a wonderful plugin on several other sites and is quite the incentive for an upgrade.
2: Award CP - Not necessarily custom, nor do I know if it's already on the forum, but a control panel for giving your own awards out to people with administrative permission would be awesome and a great way to allow for more ease in managing awards.
3: Hue changer - A plugin that simply lets you change the hue of the forum thus allowing you to create a darker/lighter environment for yourself. This would mainly be effective on the dark theme.
4: Username Effects - I believe that sparkles, flares, and several other username effects should be available in the FK shop for purchase since that would encourage more economic trade and posting and better customization for members.
5: Award Requests - A page for requesting an award you believe that you deserve. This would remove people constantly posting in the thread after every thing that thinks would grant them an award. It should definitely have a timeout for the amount you can request for so many minutes.
6: Automatic upgrades - There is a plugin that you can use with PayPal to set up automatic upgrades. I believe this would be beneficial as it would allow for members to be ensured that they could purchase and immediately begin their subscription along with confirmation that their payment went through.
7: Custom tabs in the award sections - Allows for organizing user owned awards, holiday awards, and site awards. Much easier to navigate through and much better for knowing all of the awards.
8: More contests! - Holding contests for sacred things like a free usergroup, a contest award or anything that is sought after by the community are always welcome to every community and I'm sure the rest of the community would love having more.
9: Occasional discounts - I believe that every month at the beginning of the month the upgrades could be discounted by about 5-20% and maybe also reduce the donate award costs by about 1-5% along with it.
I'll add onto this post when I can think of more ideas. These are just general ones that I'd love to see on the forum.