So you're basically asking a bunch of people to help you on a project, and not telling them what the project is because you think they won't like it? Why can't you just say what the project is? I'm really curious now.
Also, you're saying that it won't take more than a week, but also saying that you're afraid of wasting lots of time... Which is it?
I wouldn't mind giving this skull thing a stab because I think in general, a giant skull whose mouth you can enter would be a fairly useful sprite... well, okay, but I think at least some people could use it sometimes, it would make a cool dungeon entrance. But I don't want to commit lots of time to something I don't know I will like, just like how you don't want to commit time to making this weird secret thing without knowing somebody will make you this sprite.
Also, I still think turtle rock, edited, could work. Just change up its eyes, elongate it, and give it a horn.