Vilnius is a natural travel point on the way up to Tallin, not sure why it's a bonus point. Other than perhaps to force a stop in the city as opposed to just change trains/buses.

It's hard for me to predict the stopping points. There's a lot of iconic brutalist buildings anywhere from Berlin through Banska Bystrica to Budapest, even Belgrade.
I'm personally hoping for a slightly more southern route, not just a straight across shoot. I don't want to be in Warsaw until Thursday or so. I'm thinking I will end the first day at home in The Netherlands, after passing through Brussels, and then I'm hoping to catch the morning ICE towards Munich.