I like this idea very much, and I will be watching this.
Will we see Croatia brought back in fold, Triune Kingdom restored to borders it had in AH, with all the Croatian lands unified, or are we going to see something along the lines of WW2 "Greater Croatia", with Bosnia and Herzegovina? I am rather sure that the Croatian people of the time really would prefer Germans back, especially after what they have been through with Serbs in Yugoslavia, and even a veneer of self rule, with perhaps somewhat expanded borders would be enough for them to join back up with minimal fuss. Also, we must take into the account the large numbers of former AH military officers, such as Slavko Kvaternik or Emil Uzelac, through which contact could be established, and paramilitary formations formed. These people still had a lot of influence, as did the nobility, and if you dangle in front of them both the unification of Croatian lands, as well as restoration of the old Royal Croatian Homeguard, with them in it, we could see some interesting things happen.
Putting aside my ramblings, and my personal wishes, I really am interested in seeing how this Restored Austrian Empire is going to work. Maybe bring back Otto, for he does have legitimate claim, in some circles at least.
Lastly, I know it is somewhat off topic and probably will have very little direct influence on the story as such, but what are you going to do in regards to weapons, particularly personal firearms? Both Austria and Hungary used IOTL 8x56R cartridge, in essence a modified/modernized 8x50R cartridge, which enabled them to keep their older weapons of WW1 vintage in service, with minimal modifications. Other nations that used to be part of AH generally use 8 mm Mauser as standard, such as Chechoslovakia and Yugoslavia, so what is to be done about it. Greatest problem with 8x56R is that it is rimmed, making it somewhat problematic to use in automatic firearms, and considering the lessons of WW1, portable LMGs are a must, with CS zb.26 filling that need rather nicely. It is not that they could not make it work with rimmed cartridge, British Bren is the proof of that, but it is complicated and long process, and magazines do become rather curved to accomodate the ammunition.
Sorry for going off topic this far, but I am rather excited to see where are you going to take this, and I can barely wait for more.