I quite like these - in fact, my girlfriend glimpsed at my computer today and saw me browsing Lemmy.world. I explained what it was and her only conclusion was "cool. they could use a better icon though"
I've been adding icons with the mascot to my communities too - I think this touch of "branding" helps a lot with newcomers joining in, because they get confused with the multiple instances and servers, so if you make everything look a bit more uniform and connected through the icons it helps.
I like the top one best, but feel like we are missing out on some blue for the "world" concept. Would it be horrible if you were to make the green a gradient green-to-blue?
I agree that the "world" concept is missing. Maybe make the white parts green and the green parts blue so it looks like a continent surronded by ocean on a planet? Not sure if that design will be as clean though.