How plausible, or what events would need to happen for both the Austrian & German Empires to unite via a Royal marriage or otherwise, any time after 1600.
In 1600, there was an Austrian Empire, which was a part of the HRE - Austria was already member in the confederation you could consider as "German".
Later on, there are a bunch of possibilities:
- HRE unification in the 30 years war: the Austrian Archduke becomes emperor of a centralized German state
- The Revolution of 1848: either Großdeutschland (the German territories of Austria + Germany) or Großösterreich (German Austria + Hungary + Germany)
- The Bundesreform around of 1863 (Austria tried to centralize the German confederation with Austria at the top)
- Austria wins the War of 1866 and centralizes the confederation (as above)
- Bismarck is more ambitious and annexes Austria after 1871 (problem: neither Russia nor Great Britain will let this be done)
- CP win WWI, but Austria collapses and breaks apart in Hungary and Austria, which becomes a part of Germany (e. g. in "a shift in priorities)
- Weimar republic becomes stronger (or goes communist) and annexes Austria
- Hitler dies before WWII or "win" it, so Austria stays in Germany
Bonus points if it encompasses Switzerland as well.
Only possible with a real early POD... and even a centralized HRE will have a hard time if it tries to conquer Switzerland.
The other POD is Hitler winning WWII and trying "Unternehmen Tannenbaum".
What would be the consequences of this regarding the Europe, and the founding of an overseas Empire.
Austrian colonies in Africa and the Caribbean

Also, Germany gets access to North Africa.