The Lord inquired of John, who is called “Beloved:”
“John, my beloved, what desirest thou? For if you shall ask what you will, it shall be granted unto you.” (D&C 7: 1.)

This is what the Lord offers, at some point, to those who meet with Him as He confirms their exaltation. I’ve explained this in Beloved Enos. It is part of the privilege He extends to those who come to know Him.

A person could ask anything of Him. In the case of John, however, the request was completely selfless (one of the reasons he is “Beloved” by Christ). It reflects the same heart as the Lord’s.
“And I said unto him, Lord, give unto me power over death, that I may live and bring souls unto thee.”  (D&C 7: 2.)  That is, John desired this not for his own sake, but for the sake of those to whom he could minister.  He wanted to bring souls to Christ.
“And the Lord said unto me, Verily, verily, I say unto thee, because thou desirest this thou shalt tarry until I come in my glory, and shalt prophesy before nations, kindreds, tongues and people.” (D&C 7: 3.) The ministry of John would continue. There would be “nations” who would receive his prophesy. What do you suppose it means for John to be able to prophesy before “nations?” Do “nations” mean modern states, or do they mean family divisions or subsets, like the ancient tribes of Israel, which were called “nations?” Do the terms “nations, kindreds, tongues and people” have a family meaning? What family? Has the gospel been intended primarily for one family of redeemed souls all along? If so, then, how does one connect to that family? What is John doing?
The Lord explained to Peter that, concerning John, “he has undertaken a greater work; therefore I will make him as flaming fire and a ministering angel; he shall minister for those who shall be heirs of salvation who dwell on the earth.” (D&C 7: 6.)
It is an interesting question to ask what John has been doing. What do you suppose it means to be “as flaming fire and a ministering angel?” What jurisdiction does John have if he “shall minister for those who shall be heirs of salvation who dwell on the earth?” Does this require John to be involved with all who are to become “heirs of salvation who dwell on the earth?” In what way would he be involved?
These are interesting things to contemplate. All the more so because these ideas are somewhat at odds with the idea that God has finished His work and given his power to men, as we claim. Nephi disagrees with the idea (2 Ne. 28: 5.), but that hasn’t affected our views much. We’re really quite certain we have everything we need without John.