Austria, sure, everyone should see Vienna, Melk, the Wachau Valley and I guess a few other places. But know that Vienna is what it is and has been for 50 years and will be for the next 50 years; MacDonaldized. On the other hand Hungary is right on the verge of becoming MacDonaldized and other parts of Central Europe aren't that far behind. So if you want to see them pre-Western Influenced the sooner you get there the better.
Hungary is good for 10 days very easily between Budapest, Pecs, Eger, Gyor
The route from Istanbul through Bulgaria to Sofia is pretty amazing as well. Mountains, lakes, rivers, villages, horse drawn wagons. (another week). We did a repeat of Bulgaria earlier this year and drove on to Istanbul. Great time.
Then north to Romania. Some of the best preserved medieval architecture left in Europe. Great people, gypsy villages and castles. (another week) I think Transylvania is one of my favorite places in the world for getting lost in ancient history.
You can do a lot of this for about $350 a day including transportation and lodging, guides, fees, etc. All but the food.
I love Slovakia too, but its hard to work in with Romania and Bulgaria but it does excellent with Hungary and Ukraine and Austria. Yes, Ukraine. The war is a long way from Lviv and the Ukrainian Carpathians. Slovakia has some absolutely stunning castles, incredible mountains. We will be heading back on trip number 3 in the fall of next year (fishing). For that you would do a week in Austria, a week in Slovakia and a week in Hungary and a week in Ukraine.
Then you can get really off the wall. Don't know what your budget is, but if you set up a base someplace like Budapest and take bonzi trips on short, inexpensive, non-stop air trips. Budapest to Jerusalem or Moscow or Istanbul for instance. Each is about $350 rt.
These are experiences that have an expiration date. But don't expect it to be just real, real easy. Forget the train passes outside of Hungary and Slovakia. Rick has Europe through the Back Door; this is Europe through the basement window.
We spend a lot of time in the region. Let me know if I can help.
Oh, and if Hungary does make your list then do it last.