Since so many wants Austria-Hungary, regardless of how fitting it is, how about making it a disaster or event?
Can only happen if Austria has a PU over Hungary, if Austria and Hungary both have Habsburg rulers or if Austria directly owns the majority of the Hungarian cores.
This would unlock a unique governement, double-monarchy with the Austria-Hungary tag nation or something like that, which would function similar to Dutch Republic with a slider on the government interface.
Instead of ruler regency length and the republic bonuses it purely effects stats in terms of nation and ruler management so for example, if it was divided between "centralization" or "Habsburg control" and "decentralization" or something similar you would get different bonuses and such, similar to Orangists and Statists.
When leaning towards Habsburg control you get Monthly autonomy decrease plus something extra like yearly legitimacy, chance of a new heir or diplomatic reputation, but at the cost of unrest or maybe hungarian/noble rebel events.
When leaning towards decentralizing the powers of the nation(s) you would get a decrease in unrest plus accepted culture % (to reflect accepting Hungarian) but at the cost of monthly autonomy increase and maybe defensiveness or manpower/tax.
Thoughts?