....Is because you want to, not because you feel you have to just to get on the 'investment' bandwagon.
Also location as always is key.
I have recently purchased in the country,the nearest city/large town being Stara Zagora and Chirpan.

Property is cheaper in the country and usually with a good size plot. My place is in the south so the weather is better and best of all, I can drive to Turkey or Greece from my village, Zlatna Livada, in about 1.5 hours. I can get to Istanbul in around 3 hours!
Also there are less gypsies in the south as many more prefer to live near the Romanian border in the north. Property is increasing in value faster in the south because of its better climate and location to Greece and Turkey so all in all I am happy. I do not intend to sell for a long while and will look to get good use out of it once it is renovated. When im there and I fancy a nice long weekend somewhere different ill jump in my car and drive to Greece or Turkey, brilliant.
Ultimately I believe this is what buyers will want.
Bulgaria is getting enormous investment from the EU so will continue to improve over the next few years and eventually people will want to retire there or look for more from a holiday home based there. The south offers greater opportunity and a superior climate.