Tollense Valley Bronze Age warriors were very close relatives of modern-day Slavs
This is strongly suggested by the Principal Component Analysis (PCA) below, which shows that many of the Tollense Valley warriors (Welzin_BA) cluster in the Slavic-specific part of the plot. The relevant datasheet is available here.
I designed this PCA with the sole purpose of using Balto-Slavic-specific genetic drift to differentiate Slavs from Germans, except of course those Germans with a lot of Slavic ancestry, who are usually from eastern Germany and Austria. Admittedly, these Welzin_BA samples are preliminary low quality versions, but I can assure you, people who don't harbor significant Slavic ancestry never cluster in this part of the plot.
The only other ancient samples that cluster in the Slavic zone are, as expected, an early Slav from Bohemia and, interestingly, a Bronze Age individual from what is now Hungary. But we've already seen strong genetic, and indeed genealogical, links between another Hungarian Bronze Age genome and present-day Slavs (see figure 3 here).
So what's going on? Did the proto-Slavs come into existence during the Bronze Age, as opposed to the more generally accepted early Medieval Period? And did they expand from what is now Hungary? Or did they migrate there from the Baltic region? Thanks to Matt in the comments for the table below.