meh imo lulu is doing what he is doing mainly for revenge on his mother, his sister and the way he was pushed aside by the king...
he hates loosing (same as light
) so hes trying to get revenge by demolishing britannia...
now that you mentioned it...suzaku and euphemia can be the future king and queen...providing that the kind is dead...i think that the end wont be just the "destroyed britannia" but instead a reformation lead by suzaku and euphemia to pose freedom and new laws...im sure this way everyone would be happy including lulu...personally so far i dont see lulu as wanting to rule anyone...just to get even...