best bases for me to edit and also easiest were probably manuka, selestia and sio so far. they all have a lot of assets on booth. what i found out that the only key what changes the avatar a lot is basically just re-coloring. hair, eyes, clothes, just make your own palette. play with blendshapes. set your own style for your model before you are gonna start. of course u can swap bodies/heads, but even without it u can make your character look very unique by just playing. here is my freshly made sio and manuka, i do not like such a drastic changes, but i just did my own style. so i hope this helps!
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