I think about it this way:
A kingdom is a place, but a place without people is just a piece of property. In a kingdom there is a king who rules. Does he rule over the land or the people? Then there's the fact that heaven is to be destroyed, and God setting up a 'capitol' on a new earth. We cannot inherit something that is to be distroyed. Jesus sayed that the kingdom is at hand and even now is. At the time heaven was empty but peradice was full. So the conclusion is that Jesus was not talking about a pysical place, but a spiritual place. Remember that we are seated with Christ in the heavenlies. The kingdom of God is the Church, moving and opperating in the spiritual ream. God is a spirit, and them that worship Him must worship Him in spirit and truth. It is also any ground that is takin back for Christ in that earthly realm. It also includes Heaven since it exist in the spirit realm.
Anything that is under Gods direct authority is apart of the kingdom of God (everything is under God's indirect authority). Or, everything that is a part of Christ's priesthood is in the kingdom of God (not a reference to any denominational priesthood, i.e. catholic priest, but rather a reference incusive of all denominations that name the name of Christ).