From what I've seen, south Slavs are northern Europeans in terms of mtDNA, and probably about 50% northern European in terms of Y-chromosomes.

In other words, most of their ancestors were probably Slavs from the north.

I disagree. Slav mtDNA is predominantly of southern origin.

000205.html

And their Y chromosomes are not predominantly "northern European" but rather Eastern European - Central Asian - South Asian.

It's of southern origin, but it's now northern European.

Slavs, Germans and Finns have remarkably similar mtDNA.

They share a lot of this mtDNA with southern Europeans, but there is a north/south divide in terms of mtDNMA in Europe.

Chew on this...

The phylogeographic context of the southern Slavs: A mitochondrial perspective
H. V. Tolk 1, M. Pericic 2, L. Barac 2, S. Cvjetan 2, P. Rudan 2, K. Tambets 1, R. Villems 1;
1Estonian Biocentre and Tartu University, Tartu, ESTONIA, 2Institute for Anthropological Research, Zagreb, CROATIA.

Mitochondrial DNA lineages of three South Slavonic-speaking populations of the northwestern Balkan peninsula - Croats, Bosnians, and Slovenians (N ~1,200; ~370 haplotypes) - were identified combining the sequences of mtDNA HVS-I region and the RFLP data from coding region. These lineages were compared with a dataset of about 12,000 samples from elswhere. This phylogeographic knowledge base was used to interpret demographic events of the past since the peopeling of Europe. An absolute majority of the lineages found belong to the common western-Eurasian haplogroups - H, I, J, K, T, U, V, and W. Low-frequency haplogroups, e.g., N1, R, HV, and pre-HV, are present as well. Lineages, characteristic for sub-Saharan Africa or eastern Eurasia, occurred in single cases. For better phylogeographic resolution the data of the populations from different geographic areas were compared in a sub-haplogroup level, and the fraction of the identical haplotypes between population groups was determined. The distribution and diversity of many subhaplogroups reveals that the gene pool of the populations of northwestern Balkans has not gained much influence from the Near East during the Holocene. With some interesting exceptions, southern Slavs tend to have more common phylogenetic branches shared with Germanic (e.g. T2), West Slavonic, or, in some cases, with Finno-Ugric speakers (e.g., U4, U5), but significantly less so with southern European and eastern Mediterranean populations.