The Low Countries could work very well for this. I don't know how plausible it is, but considering the area was a borderland between France and the Holy Roman Empire (and the English by sea) you might be able to have a comparable evolution there. You'd need the cities and towns to gain power at the expense of the various counts and prince-bishops in the area and unite in some alliance like the Old Swiss Confederacy. I'm not too familiar with the medieval history of this area outside the basics so I'm not sure if a Swiss-style Low Countries/Seventeen Provinces is doable, although it seems promising.
If things go right then the Seventeen Provinces should evolve similarly to Switzerland and end up multiethnic. We'd have a majority of Dutch speakers (like Germans in Switzerland), a powerful minority of French speakers (like the French in Switzerland), a small minority of German speakers (maybe, they might end up Dutch or advance their regional language like Luxemburgish or Limburgish, but in any case they'd be like the Italians in Switzerland), and a declining minority of Frisian speakers (like the Romantsch-speakers in Switzerland).
Unlike Switzerland, this country would be a powerhouse of Europe and probably counted as a great power given the colonial empire it would have, strong financial institutions, early industrialisation, etc.
A state in North Africa containing Africans, Arabs, and Berbers
That's OTL Morocco and especially Mauritania.
Banat/Banatia was a strong contender post WWI even in OTL, you’d have Magyar, Romanian, and South Slavic people stuck between Hungary, Romania and Yugoslavia/Serbia.
Don't forget the huge German population, about a quarter of it IIRC.
On the same lines the Duchy of Bukovina would also work and like Switzerland is fairly mountainous. You'd have Ukrainians, Romanians, Germans, Jews, and Poles in one small country.