I've traveled pretty extensively in this region. I would say right off the bat Croatia should be off the table. It's too far away, and the plane links are not great, and ground links too slow. I would actually say the same about Albania. Its an ok country to visit but maybe on a different trip, especially since you seem to be indicating you will be back to Croatia, which is a more natural fit.
To me some country combos that might work would be Bulgaria, Romania and/or Greece. Macedonia could probably be added.
I put together a pretty extensive itinerary for 8 days in Bulgaria recently, it would be better as a bit longer though. And if you didn't want to move around so much, you could do the same itinerary but base out of Sofia, Plovdiv and Veliko Tarnovo, each of which has great side trips nearby. Although it would be a shame to not spend at least a night in the Rhodopi Mountains. There was also some great info in that thread about Romania as well, which is a great country too.
I would skip Skopje totally, possibly the entire country. Ohrid is a great town in Macedonia, but it's just so far from everywhere else that it makes it hard to incorporate in itineraries unless you have a car and a lot of time. Skopje is just IMO not worth the time. I think it would depend on how you approach Greece. If you rent a car and drive down it is possible to dog leg over to Ohrid and a few sites along the way, and then come down through NW Greece, there are a lot of parks and hikes in that area, and then maybe through Meteora on the way to Athens and then a more traditional Greek itinerary. There is also a lot to see in N Greece, but most people are not interested so I don't feel like selling it unless you have a strong interest. And especially since you haven't been to Greece, it may be better to see the "traditional" sites first. The only issue with this is returning the car to Bulgaria, I think it is like 9 hours between Athens and Sofia, there is a lot of driving there. So one strategy might be see Sofia and/or Romania, and then fly to Athens, which is easy as there are discount airlines between Sofia and Athens (maybe Bucharest too, haven't checked).