Not to be the tight guy, but I feel obligated to point out that this probably belongs on the Artwork board.
Anywho..
*ahem*
The start of anything and everything is the idea. From what I can grasp, you want a dude, an avatar, an icon, etc. That's awesome. I've always thought they support and express character and personality.
You asked what can it be. Answer is anything you want it to be. Whatever you feel expresses what you want to get across. Seriously anything. Don't limit yourself to living things or creatures. Seriously, it's all up to you, just choose yourself. Have some imagination.
The way I see it, and I could be completely wrong, when you ask "what does it need to be", that implies that you're setting a perimeter, or border on the possibilities, as if you're trying to please someone other than yourself through it. In my opinion, that's the wrong way to go about it. It doesn't necessarily need to be anything, just make it whatever you want it to be. It doesn't even have to express you directly, but whatever you want to convey.
Same goes for what it is to portray. Whatever you want it to. It sounds like you want it to portray yourself. That's just fine, and easier than you think.
Now as far as creation goes, again, back on the subject of starting with an idea, when you get it in your head, you can either make it yourself (draw, paint, digitalize, sculpt?, etc.) or you can have someone who you trust to do an interpretation of it for you, such as an artist you like, or a friend who's good at drawing, or whatever, just don't expect it to always come free.
In response to Nikolai's image, you said you want something smaller and more expressive.
Truth be told, the level of expression falls somewhere between what the artist conveys, and how the viewer or person looking at it receives it. Hell, I've seen more expression in emoticons than I've seen in some "fine art" pieces and of course vice-versa. It's all a matter of what you make of it.
For now, I think what you want to do is get an image in your head of what you want your dude to be/look like/convey/etc. and have someone help you get your mental image down on paper to start.
Frist: That's just my 2 cents on it.. er.. more like 9 cents.