You should consider to remove the spam ads in the back-end at least for paying customers. I have paid for a premuim key for any fonts and you could at least honor that with revmoving all the ads that you have inserted into the plugin.
This doesn’t look very good when you have paying customers in your system (which i have). For non paying users, well thats a different story and totally fine.
Another thing to wish for is:
1.) Multisite support .eg you could Network activate anyfonts and just set the api key in Network admin (and fonts for the system).
2. ) Supports for webfonts such as Google fonts etc.
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