Many congrats to you golfinho, I hope you enjoy the tablet and that it will be of good use to you.
I think the freedom of choice is perhaps the best approach regarding the aura, as I believe many users like the ability to easily express themselves through the color of the aura; a picture (or in this case color) says a lot more than words about a user. Some may not care at all while others enjoy the option to choose their own aura, like I do.
I do believe it is a very good idea to be able to show the NG specialties of a user, however just as a previous poster did mention, I do think a nice symbol representing an area of specialization like Flash (in general), audio or art or whatever could be displayed beneath the user icon along with the total amount of posts and such, if the user fulfilled the requirements for a such icon/symbol/tag/stamp. These icons could in turn be color-ranked too, and with ease tell a user in a second approximately either how many submissions the user have or what the average score on the submissions are.
Either that or a frame around the user icon, and in so keeping the aura as it is while the frame display the user's specialization. I personally consider the symbols beneath the picture to be a better choice instead of tampering with the aura too much.
You should advertise your need for help with the icons a bit more, 28k submissions to iconify is a lot of work. Also, I'd like to suggest an option that allows me as a user to preview my message before posting it.