Luke 21:31

“Even so you also, when you see these things coming about, know you that the Kingly Rule of God is near.”

So in the same way when they see all ‘these things' coming about they are to know that the Kingly Rule of God is near. Here it is clear that the future eternal Kingdom is in mind. In context ‘these things' refers to the signs prior to the coming of the Son of Man for in Luke 21:36 it is ‘these things' which can be escaped from, whereas the coming of the Son of Man is inescapable. Compare also Luke 21:28 where ‘these things' cause men to look up and lift up their heads in anticipation of His coming. They cannot therefore include His coming. ‘These things' are clearly therefore the indications of the possibility of His coming. This in fact agrees with Mark 13:4 where ‘these things' refers to the destruction of the Temple and what will accompany it. Thus the main idea in mind here is that when they see the destruction of the Temple and the leading captive among all nations of the Jews, following on after the other ‘things' that He has spoken of, they must then recognise the ‘imminence' (the possibility of it happening at any time) of the establishment of the eternal kingdom. Nothing will then be required to happen before His coming occurs, although He does not know when that will be (Mark 13:32).