Hi June, I'm a little confused by your question (it doesn't take much to confuse me). . So your total trip is 10 weeks, including the 10-day tour of Romania? If so, since you do have Budapest on your list, I would either put Budapest at the beginning of your Romania tour or at the end, because it's so easy to get there from Bucharest or Cluj.
I'm not sure if you're looking for additional places or if you're looking for what order to put the above countries in. So I will put what I think is a logical order for the countries, and let me know if I'm totally off base here.
It sounds like you are starting in France. After that, it would make sense to go to Dusseldorf in Germany. From Germany, head to Prague, then on to Budapest. From Budapest, it's a quick flight to Cluj-Napoca and the start of your Romania tour. Your tour ends in Bucharest, so fly from Bucharest to Croatia and then travel to Slovenia.
Not counting your 10 days in Romania, that would leave you with 7 countries in 60 days. It's a nice mixture, and really no backtracking at all. I would love to be on a trip like that. 😊
I agree that there are many places where you would want to fly, and one of those is Romania. While they do have trains there, they are not the fastest method of transportation, and there are flights going all over. I would definitely fly into Cluj and fly out of Bucharest to wherever you go. France and Germany, of course, have faster trains, so you might want to stick with that for your mode of transportation there.
Having said that, is that what you are looking for? Or are you looking for possible substitutions for your list here? Or additional countries to travel to? I know that 10 weeks is a long time, but I think that what you've got now makes a complementary list of places you could easily see during your 10 week period.