Is Mario Kart 3DS going to have more caracters?

Not much is known as of right now about Mario Kart 3D. Nintendo is trying not to reveal to much information about any 3DS games this year, so they won't build up to much hype. They still want the Nintendo DS to do good this holiday season, so information will be rare. You can expect more information by late January 2011 though, that's believed to be when more information will be released on many nintendo 3DS first party titles (Paper Mario, Mario Kart, Legend of Zelda, ect). But, the little that acually is known is:

Confirmed Characters:
Mario
Luigi

And the stages that have been shown in the screen shots aren't titled yet.

One thing I would like to note though, is that in one of the screenshots nintendo revealed there is Miis in the background cheering Mario and Luigi on.

If you would like to see these screenshots for yourself, you can view them on Nintendo's E3 website, Here. Scroll down and you will see "featured titles, nintendo titles, more titles" Click on Nintendo titles, and on the right you will see a list of 7 Nintendo 3DS titles. Click on the third one, which is Mario Kart.

Hope I helped. ^_^

When we gain psychic powers, we'll let you know.

There is going to be a course through Peach's castle.

By "through Peach's Castle" do you mean inside Peach's Castle like in Bowser's castle or around the castle?

Yes it will have the game and it is awesome there is new power ups 2