Challenge and What If : Spartacus and Crixus take Rome.

Be free, albeit materially poor.

I would imagine armed slaves who've defeated Roman armies would be in high demand as mercenaries by the Germanic chiefs.

Or they might decide to join with a strong Germanic chieftain. This could lead to a brief period of German unity/power, but it would probably collapse following his death.
 
How did Rome not raise one army after another against Sulla, Marius, Caesar, Octavian?

They could have, but didn't really need to; all the above were Romans, as were their opponents. Whoever won Rome would still be ruled by Romans, which was fine with them. Whereas Hannibal and other foreigners were opposed with all the force Rome could muster (sometimes successfully, sometimes not).
 
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