Yes, adding the outer planets would be cool, but what I would really like is adding their moons too. The main ones of course.
Actually, the gas planets are supposed to have no solid ground, and their gravity is such that you would not be able to lift off from them (not even talking about their super dense atmosphere!). So they wouldn't be that interesting in themselves.
And adding Pluto would be cool too. I really want to visit Saturn's rings and Pluto!