How to monetize over a million users?

An app that I built as a fun weekend project grew to over a million registered users : Widgetbox.app. 2.7M widgets were created and I get an average of 2M hits per day (mostly from embeds - so no advertising potential).

For over an year I just offered the widgets for free and then introduced a premium subscription for 2.99$ / month.

I still keep getting new free users but the conversion is low. I would appreciate any ideas around improving the conversion rates and adding more value to the end users.