With a POD after 1815 have it so that by 1900 German speaking nations (not just Prussia or Austria) are in the worst possible position (size of territory, economically, even perhaps population), please be willing to back this up with intelligent arguments, no ASB. Thank you.
You need a French-Russian alliance prior to German unification 1866/1870.
1) no French involvement in Crimean war 1854
2) Prussia and Austria support Britain vs. Russia in 1854 to some extent
==> Russia is hostile towards both Austria AND Prussia
3) 1864 during the Danish-German war, both Russia and France intervene and carve up large parts of "Germany" and utterly destroy/disintegrate the two potential threats (Austrian Empire and Prussia).
==> results:
a) Russia and France are the unchallenged rulers of Europe
b) several small German states, partly independet, partly French/Russian puppets; later industrialisation, much more emigration to North and South America, any attempts of "unification" are prevented by French/Russian intervention later on