The Balkan have always been a peripheral point of interest for both regional and global powers until something happens where war or other issues are at the verge. Today, we have the US, China, Russia, EU and Turkey all in some shape or form attempting to gain a foothold, influence, etc with the countries in the region. The EU is the biggest investor, and all countries are working to join the Union at some point in the future, therefore, it makes sense that long term the EU (if it continues to integrate further) will have the ultimate influence and leverage in the region. This of course is dependent on the EU surviving in the next 15-20 years. Let's take a look in more depth on how some of the key countries are currently exerting influence in the region:
Turkey is very present in Bosnia, Kosovo, Albania, Macedonia through economic investment, cultural exchanges and their big budget to restore old ottoman places like mosques, bridges, etc. They are also opening a lot of private universities- for example in Bosnia they have a few and they send many of their citizens to study there- further building relationships with countries in the Balkans. They also signed a free trade agreement with Bosnia, only second such agreement for Turkey. This is beneficial for Bosnia given the massive Turkish Market, that along with Bosniak political elites being friends with Erdogan on a personal level gives Turkey and the Bosniaks mutual trust and support from both sides. Turkey is also investing big in infrastructural, for example spending billions on the Sarajevo- Belgrade highway system.
China, just recently started to invest heavy in the region, for example in Bosnia they are investing nearly 700 Million Euros to upgrade a coal plant and Bosnia just cut visas for Chinese citizens wanting to travel to Bosnia and vice versa which brought over 300K Chinese tourists last year alone. China is also looking into other areas of investment, airports, railways, ports, etc. and they are due to be a big player, however if the countries in the region join NATO and the EU, Chinese economic leverage will void their political leverage because the EU and NATO will become dominant.
Russia is exerting it's influence mainly through Serbia, therefore limiting their influence to one country which means that they don't really have the ability to be a big player in the Balkans ( Serbia is one small component of the greater Balkan region).
The US has minimal investments in the region, decreased their troops but is a key guarantor of peace ( backing the Dayton peace accords in Bosnia which is the key for peace, if stuff goes wrong in Bosnia you can almost guarantee a regional conflict involving Bosnia, Serbia and potentially NATO- Croatia) and they help push countries in the region towards EU and NATO. NATO, is the most important tool the US has in the Balkans, by accepting more members it eliminates the possibility of conflict in the region, and potential future adversaries, pushes the countries further away from Russia as well.
Ultimately, long term the EU, currently probably an equal share between Russia, Turkey, US and China. I also don't think we will have only one key global or regional player but rather multiple.
