Make the default avatar show any and everywhere you can (Zachery/GeekyDesigns has a default avatar mod btw and one or two others too if I recall correctly) but ensure it shows the default avatar. Now take that default image and make it something funny but also "meh" and by that I mean - Make some really nice custom avatars, ones they will see and go "ohh wow I would like to have one of those" and then the default one is a silly frown smiley face w/ a mustache or something, it would scream "replace me" but still not look odd on the forum as it should match the style in a sense but the new avatars would be spot on and make them want to replace it.
Other ways to do this but I've found that making them see a "meh" image inclines them to replace it sooner rather than later and the default white/grey silhouette does not prompt you to, want to do anything but read a drivers education manual because it reminds you of a street sign related to pedestrian crossings

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Edit: Moved from Community Review to Community Lounge

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