As I have previous said, from reading the Fourth Gospel as well as the core sense of Christianity, I do not believe that Jesus Christ promised a Second Coming. On the contrary, I believe Jesus fully achieved everything he incarnated to achieve in a cosmological sense - in terms of changing reality; and that since ascension His role is to guide all who ask for His help, by the Holy Ghost.
But a familiar worldly-Messianic project of redemption of this sinful and suffering mortal life, of Jesus returning to take-up kingship of a New and Purified this-world, seems to have been introduced into Christianity at an early stage - and continues.
For such redemption to happen, this-world would need to be remade, in such a way that all that is evil and of-sin - including death - would be removed, purified, transformed; and only Good remain.
This might be envisaged to happen all-at-once at the second coming; but there have always (I think) been some who saw this happening gradually, incrementally, a bit at a time.
I first understood this in listening to lectures by Stanley Messenger in which he expounded the ideas of Rudolf Steiner. In this explanation, the spilled blood of the crucified Christ entered the substance of the Being that is planet earth; and initiated a process of transformation that could be explained in terms of alchemy and homoeopathy; and would lead eventually to the total redemption of earth and everything the dwells here. In the end (as I understand it) there would be a complete integration of all, into full accord with divine purposes.
I have also come across what seems to me a variation of this basic idea in Philip K Dick's Exegesis; where he is discussing Jacob Bohme and AN Whitehead. PKD's version is that this reality began as dominated by chaos and continues as entropy. God began creation in this context; and there has since been a process by which creation gradually overtook chaos; in which negentropy incrementally overwhelms entropy... Until either at or by the Second Coming, the process is completed and all that is evil, destructive - all suffering and pain - is transformed into Goodness and Happiness.
My above summaries are themselves of secondary sources, thus unreliable as to detail - but I offer them as the kind of thing that would need to happen if this mortal world were to be saved, redeemed, made into Heaven on Earth. In other worlds, all that is evil, all destructive change, all death - would need to be transformed into harmony with divine creation.
And this transformation is regarded as something that will happen. It is not a matter of choice, but of processes acting-irresistibly-upon Beings. Evil and Sin are eliminated by being made good.
Now - I regard this as both impossible - because evil cannot be made Good; and undesirable to Christians - because salvation must be chosen.
More fundamentally; I do not believe that this is what will happen! I think it is a mistake to suppose that Jesus said he would make Heaven on earth, or by processes incrementally to transform mortal to immortal life.
The real situation is much simpler; which is that evil and sin will not because cannot be eliminated in this world and mortal life; which is why we must die and be resurrected to enter the state of only-Goodness: we must be born again.
Those, and only those, who choose resurrection (and allow/ embrace the necessary transformative changes to themselves) will be added to Heaven, will join Heavenly life - leaving-behind their sins and all evil; and from thence, living only by love.
This mortal realm with its sins and evil will be left-behind; as a place for those who choose to hold to their sins and evil, and those who choose not to live wholly by love.
Such must happen; in order that Heaven be possible; and it must happen because eternal Beings must dwell somewhere.
And if Beings do not want to dwell in Heaven, then they will remain in some part or variant of the mixed-world of this-world of mortality - where entropy and creation contend.