Empires only experience de jure drift to entire kingdoms after absorbing their entire de jure territory. I think I see some realm boundaries in Wallachia there. You're missing something. I just had a playthrough starting as duke of Karvuna where I formed Wallachia myself while still a vassal of the Byzantine Empire. You could probably use the same trick here; if you create that Kingdom and give it to a vassal, any remaining territory there will be his de jure territory and he can either vassalize independent dukes there or press de jure claims himself. You could grant it as a viceroyalty to avoid it having the Imperial Elective succession law, since that doesn't work too well in small realms (requiring commanders/marshals to be landed and all that.)