I think it would be a great feature to have the ability to choose (from a list of pre-approved, optimized/safe avatars) what would show as our blocked avatar, rather than everyone being reverted to the same default gray robot.
This way not everyone looks the same when you feel the need to run in safe mode for performance/safety reasons, but also everyone will still be optimized/safe for those situations. Not to mention, the chosen replacement could be of similar height to the one that is being blocked.
A good example of a list of pre-approved avatars could be the Featured list or the VRC Public Avatars, so long as they were all optimized properly.
Another idea in the same vein as the one mentioned above was to simply have a chosen optimized avatar from an approved world or even just one that is well rated through the SDK that you can set as your Optimized version, serving as an in-between to being a fully blocked avatar and a partially shown/fully shown avatar.