Almost every assertion we make only begets a new question. If we claim that heaven is a spiritual existence with God, then what is hell? We can't really call it a spiritual existence without God because God is omnipresent, existing in all places at all times. Even the unsaved will spend eternity with God then. Hell may be better described as an eternal existence against God, an eternity of living in unchanging, irrevocable opposition against that which you can never change, the overwhelming presence of God's omnipotent Spirit.
Mankind's ongoing problem is that we are born as creatures of the flesh, and as such, we are in natural opposition to things of the spirit. We live in the flesh which goes against the grain of the spirit. And since God is the One True Spirit, we end up in a natural, lifelong opposition to Him. This is the battle between spirit and flesh, between good and evil that we are all destined to fight. It is an internal Armageddon going on inside all of us. This battle going on within the hearts of men could well be in perfect accordance with God's plan for us. This battle in and for the hearts of men may be how all flesh is eventually brought into willing subjugation to the greater Spirit, the Spirit of the one, true and eternal God.