Some people may use the phrase "Kingdom come" to refer to heaven or forever
I will be working here until Kingdom come
however there is an alternative definition which refers to the next world. Both definitions provided in the link are badly referring to the Holy Bible and the Lord's Prayer. If the many Internet websites out there are correct on the origin, it is strange when you look at the origin.
The origin is indicated on these websites as being from the Lord's Prayer from the line
Your Kingdom come (English Standard Version)
Thy Kingdom come (King James' Version)
which is asking for the reign of God be fully established
Now for your reference to Kingdom come, without knowing what songs you are referring to, you could be referring to a hip hop slang where Kingdom come is meaning new home
Yo Jay's Kingdom come is da illest!
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Looking at the full lyrics of the Imagine Dragons song Demons, they are referring to hell (the opposite of heaven) - Kingdom come being next world - as they are talking about idol worshipping and greed.