I don't think 'invade' is the concept you are looking for. The Balkans were fundamentally different from the Baltic in that there was an established power structure with clear territorial claims in place. An attempt by a band of monastic knights to just walk in and set up shop would have been the two-weeks-and-an-hour variety (as in, the Hungarians laugh for two weeks and mop them up in an hour).
Of course, even in the baltic they didn't exactly do the terra nullius thing, and it is quite conceivable that a local ruler (Hungary springs to mind, actually) would be happy to integrate them into his power system. They did, for a while, offering them lands on the Cuman frontier. Fighting those pagans would have kept them busy and happy enough,. I guess, and been a boon to the military power of the kingdom. In the medium term, it might mean a solidly Catholic and partly German-speaking Walachia and Moldova. Interesting down the line - perhaps an area of South German Lutheranism that survives the counterreformation under Ottoman protection... Anyway, it's probably not what you are looking for, but evenn in its best days, the Teutonic Order on its own stands no chance against the Balkan kingdoms, let alone the Ottomans.