I must admit that all of those Bible passages do seem to hint that John could still be alive, somewhere in the world. I had always wondered if it was true, too. They also hint that there might even be more than one that would ‘remain’, because Jesus refers to “some who stand here”. But, I think Jesus was most likely speaking about some of them seeing the Kingdom of God in visions. John certainly did when he wrote the Apocalypse, and Peter also received visions, such as the one about following the Old Law about eating things that were considered to be ‘unclean’. I have no doubt that they, and others, probably had many other visions that were never recorded in official documents. St. Paul also, clearly had visions, such as the one where he was converted. Those things were probably much more common for them, than they have ever been at any other point in history.
All throughout the history of the Catholic Church, there have been many other great Saints that have had a lot of visions (as well as many people that claimed they did, though they were proved to be false). It’s something that is fairly common, and one of the ways that God remains in direct communication with the Church. One of the most recent ‘approved’ visionaries that I can think of was Padré Pio (he died in 1968), who always seemed to be in an ecstatic vision during the Consecration, at every Mass he said. He also had several visions of Jesus and the Blessed Mother, that he wrote about in his letters to his spiritual director.
Every night, Padré Pio was physically attacked by demons (they hated the fact that he was saving so many souls

), and the other Friars could clearly hear them bellowing and beating him. The next morning he would often have fresh bruises where the devil had thrown him against a wall, but he just laughed it off. The devil also tried, many times, to come to him as an ‘angel’ or some other holy Saint, but if Padré Pio suspected that it was not real, he would ‘test’ the vision by asking it to pray with him, or by making some kind of statement praising the Glory and goodness of God, and the vision would instantly vanish…
POOF! The devil will never praise God or any of the Saints, nor will he ever say the Our Father as it’s properly said, or make the sign of the cross, correctly. He hates all of that ‘holy’ stuff and will run away, every time.