I'm very disappointed by the absence of Switzerland, France, and Italy, which could be excellent cross-border countries.
Actually when it comes to cross-border routes, Czechia would actually be great as well.
Especially the Dresden-Děčín one through the Elbe valley is just absolutely stunning and also has a lot of traffic.
Ah and btw the new czech route is actually near the german border, even though it doesn't cross it.