Hmm, have a look here at the structure of the admin trees https://www.twcenter.net/forums/show...tal-War-Guides
There should be one other level above Hungarian Forced Assimilation, the Territory Cession. However, will need to check to see if there is any other requirement to get that level.
One thing to note is that Austria and Bavaria share the same cores, so basically there is no difference between factions. Whereas as Hrobatos said Wallachia and Hungary are completely foreign to each other. Have you tried conquering Hungarian or other regions with Austria? You will have the same situation.