4 planets is easy yet boring, right? Why not add a 5th (6th if you add Luna) planet into the system? Something... Closer to the Sun than Droo!
My opinion is: model it after Venus. Like Smenus from SR1, there's a thicc atmosphere, high ambient temps and an escape velocity 5% less than Droo.
And yet, it is still not enough. Another planet modeled after Saturn that basically creates a death zone in the rings.
Moons for each of the three other planets. Deimos, Ganymede and Triton. For the Saturn model it will have a Titan model.
What are your opinions? Let me know in the comments below!