Essentially the idea is Spartacus goes to eastern Germania specifically the polish region with his army and founds a kingdom what would be the consequences of this considering a lot of the slaves were quite educated.
Once outside immediate danger (wherever and whenever they considered it safe) a good deal of slaves would probably go their own way trying to make it back to their home regions. Not all of them would choose to return home, since it was really likely that their homelands were conquered by Rome (Spain, Greece, Italia), that they were born a slave without a home, and that it was near impossible for large groups to find help while staying hidden from the Roman authorities. As for the Alps they'd probably go east through the Isonzo valley, the traditional route of invasion in and out of Italy due to ease of travel, or risk a good deal of them dying in the Alps. A cynical move and one quite frankly outside of his control was to let Crixus' smaller army of slaves pillage and distract the Romans in southern Italy when Spartacus escaped.
I'd like to point out, that Rome was formerly a simple village of castaways and bandits at its founding; should Spartacus manage these slaves well he'd have a clear military superiority over the local Germanic, Dacian (difficult since this was when Burebista united them and starting expanding), or Illyrians tribes depending on where he went. The main thing was to keep moving far enough to be a bother for the Romans, who will eventually lose interest as they devolve into civil wars. The hard part would be to rule, funny thing is the slaves were probably as skilled if not better than the Romans at this point; considering how much the Roman Senate delegated to them.
For reference here's a map of the Empire decade after, the changes are the inclusion of Pontus, Judea, and the gallic wars:
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As for the society that could come up, I'd imagine a highly martial society; after all Spartacus and the gladiators trained all the men, women, and children for war while in Italy. The society would be more gender-equal since women are soldiers as well, whether they continued this tradition or not is debatable but since they are displacing populated lands they'll probably keep it for at least a generation out of necessity.
By simple contrast the new polity would be more equitable and egalitarian; there aren't hard institutions of power yet but in comparison to the soon to be emperor who owns 50-70% of all the wealth with senators and the rich that owned the rest it will be more equitable. Adding to the equality would be the fact that all the initial citizens are militarily trained if armed to varying degrees. By necessity they'd have to be rural, most of the ancient world was with the exception of Italy later. At the same time they'd need to be able to coordinate their efforts in war. Dunno maybe a confederation if centralization fails?
Who knows, maybe they'll stratify into a new slave owning aristocracy, some sort of military republic where citizenship comes from service, it really depends on their initial decade or so. One things for sure; unless they are in Dacia they won't be on a lucrative trade route/resource but still relatively well off-especially compared to the average impoverished Roman citizen. Yes the North is colder, but the soils of the Danube and Germania (not Swabia) are black and rich; clearing them would be a gradual process spanning decades yet definitely doable and rewarding. Considering the lack of fortifications in the North the border was porous and there will probably be inform trade to the locals and Romans along the frontier.
In terms of slave policy it is really questionable, there's still a place for slavery and yet they never explicitly declared against it; yet I'd imagine that the ex-slaves would have more moral qualms about it compared to most of the ancient world. As for Roman policy it'll be hostile, at least until the civil wars and the current generation forgets. I don't think they'll instill uprisings or accept slaves overtly just to maintain relations with Rome.