To be honest, there's not much to be said for Shellhead. He has not, to my knowledge, been officially mentioned in any context, fictional or otherwise, since Ken 10 aired. If this is about rumors that he'll appear in T.G.I.S., I must admit that I doubt it. Most recently created aliens were known of ahead of their debuts in the cartoon, and while whatever happens in T.G.I.S. is a well-kept sectret, the fact that we've heard nothing about Shellhead seems evidence against his appearance rather than for it. On the subject of Ken 10 aliens, I think that a major part of why Toepick got to appear was because nothing was known about him, other that that he is gross. The creators got some freedom with him and his powers could be a surprise. Unless Shellhead has powers that we don't know of, and Kenny made it pretty clear that he probably doesn't, I don't think the creators are inclined to bother with him. As I see it, Sandbox, Atomix, and Snakepit are more likely to appear than Shellhead.