'The' is called the 'definite article' in English grammar, as opposed to 'a' and 'an' which are called the indefinite articles. So: "He broke a window," could mean any undefined window in general, but: "He broke the window," means that there is a particular window that has been defined. In this instance, I would say that in English the concept of 'future' needs defined to sound correct. I.e. :"King of the future." or alternatively: "King of future times." "King of future," sounds like an unfinished idea but is not necessarily incorrect - such is English! With reference to the idea of 'king,' Leaving it as 'King of...' means that this person could be one of any number of kings. Expressing it as: "The king of..." means that this person is the one and only king. Hope this helps.