The unification of the various entities in Germany really started following their consolidation under the mediatisation program of Napoleon following the War of the Second Coalition in 1803. Under the Confederation of the Rhine, the German states then more of a shared experience and thus shared national identity. This would lead to greater economic cooperation and eventually the Prussian-led unification against the Austrians first and later the French.
The challenge is with a post-Napoleon POD (that is, after the HRE's dissolution at the hands of Napoleon and its consolidation into fewer states) prevent the German states from uniting into one Germany. Further consolidation is allowed, but there must be at the minimum 4 states present: Austria, Prussia, and two others, bonus if you can get more.
How would this affect the German identity? How about European diplomacy and nationalism? Would this affect the Italian unification process?