I've been mostly keeping out of the rumors until it's announced. But I know the dual portable/ Home system rumors.
I think there will probably be a cartage slot, like the 3DS. Nintendo, Sony and Microsoft are still married to the retail stores who want product to sell, preorders to place and traffic in their stores.
Who knows, maybe it will have a disc drive docking station or something, for Wii and Wii U games, or it will be able to run eShop downloads at the least.
At least this means both eShops will be merged into 1, more like Sony.