Think outside the box: bring in users from outside the itch/game dev community. Can something be done with Facebook? For example, an "app" or whatever they're called similar to what MindJolt (or other portals) does on Facebook. Take a number of HTML games that are already on itch (with developer permission of course), put them on a Facebook "itch" app, maybe add some ad-revenue to it, and have the games link back to the broader itch site in an attempt to grow the itch audience.
This seems a bit different than the other suggestions here, but I think we need to think about bringing in new users to itch, and not necessarily through exclusivity or telling people how much better the itch version is for the dev than the Steam version.