Were the Nazi racial delusions explicit on Slavs in general?
They sure went totally crazy about Jews and I also realise the plans about near extermination of intellectual Poles and Russians to leave the rest as slave Labour, but other Slavs were readily included in the "German cause" although not as peers.
Anyway, I think we could be much closer to your situation, if we could "create" some situations in 19th century and early 20th century, where Slavs and Germans had a clear common enemy, and where the Slavs had been an obvious help to the Germans. Like the Poles bailing out the Austrians in 1683 before Vienna, but this time against the French.
Could it be the war of 1870 initially going terribly wrong for the Germans, and then some Slavs entering the scene as the Cavalry in the last moment? The problem is however, that by 1870 all Slavs would be included in either Prussia, Russia, Austria or Ottoman Empire.
So we also need an Independent Slav nation of a certain combat weight - Poland - and one created without stepping on too many German toes. Quite a challenge - we certainly need some brilliant dancers here!
Could we have the Poles defect from Napoleon in time, how about just before or during Battle of Leipzig - and be the ones who save Blücher from being wiped out? It still will be a long way before we have Prussia, Austria and Russia agree to give up territory to create an independent Poland, but I suppose both the Prussians and the Austrians in early 19th century were deeply dependent on British subsidies and all could find a reason to not let Russia grow too big. Now, what if we also have Alexander disgrace himself, like being captured by the French at Leipzig, which came close in OTL - and be liberated by Poles! The last, most effective but also very pleasant part of the plan, is having an explicitly attractive Polish girl thoroughly seduce King William Frederic of Prussia, the Crown Prince, the PM and the Queen.



General Blücher would prefer a French Grenadier, but I guess there would be plenty available of those after the war.*
*Among contemporaries there was a serious concern over Blücher's mental health. He was so have said to have claimed that he was pregnant and was to give birth to an elephant and that the father was a French Grenadier...