There are three emediate effects.
1) Slavs do not become divided allowing for a strong central european slavic state to counter germanic expansion eastward
2) German part of HRE consolidates sooner and creates a distinct identity unrelated to Italy and undistracted by Italy
3) Italy is thrown into a new half a century of darkness and widespread wars with a foreign element separating them from their romance neighbours in southern France. Much will depend whether Magyars remain Ugro-Finns or become Indo-European Romance speakers.
1) Was there a Slavic state there in the first place? Just because they're not divided by Hungary's presence doesn't mean they unite.
There was this one, but it would still likely be destroyed in this scenario, as the Magyars are going to have to go through it at some point to get to Italy.
Umm what? Are we looking at the same map?And if the map is correct, it still isn't All Slavs, even not counting Russia.
Umm what? Are we looking at the same map?
A hypothetical surviving Greater Moravia, or a successor state would have more interest than OTL Hungary in the affairs of fellow (west) Slavs, which would, if not lead to expansion to the Oder, then at least stop German expansion eastward. They would certainly fare better at converting the remaining pagan Slavic populations than the Germans and Danes.
A union with ATL Poland, considering OTL trends with the Polish-Bohemian relationship, could be quite possible and even likely.
And if the map is correct, it still isn't All Slavs, even not counting Russia.
No, a state encompassing all the Slavs would have been thoroughly ASB at this period in time. But you asked if there was a Slavic state in Hungary prior to the arrival of the Magyars, and there was: Great Moravia.
Yeah. I presume - correct me if I'm wrong - what-became-Poland is still uniting at this point.
Looking at the map, OTL Hungary looks to be on a direct line between Ukraine (isn't that the way the Magyars came into Europe?) and Italy. So, I guess the question we should be asking is, what could have forced or induced the Magyars to leave the Pannonian basin and go west?
Great Moravia gets its act together and kicks them out?
Someone in Italy invites some Magyar mercenaries in, and they send word to their friends about what a nice, rich place it is?
Byzantines invite them in, to reduce pressure from the Lombards?
Other ideas?
WHEN do they migrate?