Romania and Bulgaria
I'd recommend visiting Bulgaria and Romania. They are both extermely safe countries outside cities and very bike friendly, in fact the locals are friendly full stop, don't believe what you read in British tabloids.
In the countryside you will find yourself overtaking horses and carts, when you ride through villages expect kids to wave and shout (in a nice way). Wild camping is acceptable as long as you use common sense. Wild dogs are very common in both countries, can be a bit disconcerting to hear a pack of dogs howling when you are in a tent in the middle of nowhere but dogs in Romania are mostly harmless and in Bulgaria are innoculated then released with tags on their ears.
The only downsides are the road surfaces - so bad that riding at night is not recommended and the standard of driving is jaw droppingly bad.
Road signs in Bulgaria are in Cyrillic, so expect to get lost without a Sat Nav. What looks like a major road on a map may become a dirt track without any warning.
A GS is definitely not needed but long travel suspension would definitely be a bonus.
Currency can be a headache- you will have trouble geting rid of Bulgarian Levs once you leave the country, the same to a lesser extent for Romanian Rons.
I went through SE Hungary to Mako then across the Danube, onwards to Sofia and back over the Russe Bridge towards Bucharest before heading over the Carpathians and the Transfagarasan Pass, through Transyvania and back through Hungary/Austria/Germay. It is possible to go through Serbia but you will need to buy insurance at the border and when I was planning it in 2009 you had to report to a local Police station if you weren't staying in a hotel so I gave that one a miss, possibly my loss.
Border crosings are manned but no problem, just a cursory glance at docs, no toll rodas that I found. You will have to purchase a Vignette to get through Austria on motorway but it is not expensive.