Why stop at spheres. We could have Fred use a asteroid/nebula editer that edits the shape and limites of the asteroides and nebula. Just use a check box to say you want asterod and/or nebula. We could have the effect (gradual or all at once) either way as well, just have a ratio button.
We could also expand what effects nebula has on ships(ST Armada type nebula); such as em interference, Radar interferince, radio interference, system interference (engine performance reduced, weapons less effective, shields less effective, etc.), damaging, etc. Some could be the inverse improved performance.
This could allow unique game play as each race is effected differently. Older and more advanced races could have technics for channaling the different nebula properties.
This would make it great for gas giant missions.
the effect could also be used for an atmospheric missions, the Inferno 2 Release if I'm right and the Star Fox Mod.
edit:
I just though about something else. we could have packets of volatle gases that ignite when weapons, engines or even contact with the hull of a ship.
And think about having a mine field in a sensor inhibiting nebula. that would be a very nasty surprise to walk into.