I think neutral planets should have a settable strength in the game parameters. This way SP players can have more of a challenge.

Perhaps like this:

Puny: a few ships, perhaps few turrets

Normal: some ships, probably turrets, maybe hangars and a frigate factory

Strong: ships, turrets, a factory, probably hangars and repair bays

Very strong: many ships,, factory, hangars and repair bays, maybe a capital ship

Super strong:all that, plus a capital ship factory

Unfair: All that, plus a starbase 

They should also make their own money and improve over time. If they are attacked but not conquered, they should work up their defenses again if they have the time to do it.

Another option, which should be settable (enabled/disabled), is the option to have AIs spawn. Say I am on a huge map. As the neutrals work up their economy, two or more neutral planets could ally and create an empire. An AI would be created mid-game and take control.

It would only happen if planets are neutral for a long time.