I kind of see what you mean about the panda Knuffel, Firn. Mm... maybe making his face a little wider would help? Or changing the black and white proportions? I mean, I still like him, and would get him, but since you asked, I just had to say something.
Well, Solar spirit is just kind of a vague term, sort of like the Lunar spirit is. Considering that the sun is just another star, you could use a star pattern or something like a sunburst (the circle with a rays around it) for outfit design/accessories. Yellow and red would probably be the most obvious colors, though white or blue could work as well. I'd probably suggest making them male since the Lunar spirits you made are female, maybe with a wing design that resembled flames.
Now spirits such as wind, fire, earth, water, plants, and the like are much easier to define. Earth spirits are typically depicted as gnomes, and they can be a lot cuter than those hideous lawn ornaments.
Water is shown as creatures like the Greek myth Nereids or Undines. Undines are kind of a cross between naga types and mermaids, I think, with webbed ears and possibly something like a trident. The Undines are female, just like the Nereids, by the way.
Fire is often identified with the Ifrit or Efreet spirits. Those are male, well muscled, and kind of related to the djinn/genie folk.
Plants are, of course, the Dryads, and I remember that you already made a tree Knuffel before, so that covers that one. But hey... maybe someday you'll make another.
Wind are the sylph spirits (but I think the djinn sometimes have a wind connection too, instead of fire). I've seen them pictured as both male and female in different places (though female seems to be predominant). They're similar in a lot of ways to pixies and fairies, but you could make them have a different feel if you favored more of an oriental look with their clothing. Maybe have them carry a bow and quiver.
I know I'd buy any of the above elemental spirits if you made them.
You know, I'm not sure I'd buy a zombie Knuffel unless it were really, really cute (past experience tells me that you can make anything cute, so you never know).
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