arrgh... Dali makes me want to puke in my hand.

I think if you have an idea for a character you should model it. I agree with Mastone that its a bit too specific.

If you want to do a short challenge make it something simple like model a piece of fruit to look realistic. If it were 2 weeks i would suggest model a head, which you should also make as realistic as possible.

Making things up and modeling them is fine if you have some time and experience to do it. I think as most of us are starting out in maya, so realistic modeling using proper reference images is the thing to do. Its pretty difficult to give criticism on something completely fabricated.
As i've not been doing maya for long i haven't got much in my portfolio and I'm sure some people are in the same boat. If you are looking for a job in this industry then modeling something real is what you should be doing. (this may not be true for games.)