The transfiguration most certainly was not a vision of the future. It would have been an absolutely pointless vision. We don't need to look at Luke's account to know that, but let's do it anyway:The NT does not add anything to the OT and I hope that's not what you meant. It may reveal something that was hidden, but the foundation is still there in the OT. There's no addendums in other words. All the NT scripture came from the OT. Moses and Elijah are still awaiting resurrection the way the rest are. The transfiguration was a vision of the future.
Luk 9:30 And behold, two men were talking with him, Moses and Elijah,
Luk 9:31 who appeared in glory and spoke of his departure, which he was about to accomplish at Jerusalem.
Luk 9:32 Now Peter and those who were with him were heavy with sleep, but when they became fully awake they saw his glory and the two men who stood with him.
Luk 9:33 And as the men were parting from him, Peter said to Jesus, “Master, it is good that we are here. Let us make three tents, one for you and one for Moses and one for Elijah”—not knowing what he said. (ESV)
A "vision of the future" in which Moses and Elijah "appeared in glory" and spoke of what Jesus was "about to accomplish at Jerusalem"? No, that was happening then and there. And that is consistent with God being the God of the living, not the dead.
Well, they're not dead, are they. In using John 3:13, you had better keep in mind that Elijah did not die. I have no idea what the relevance of Heb 11:13 is.Moses and Elijah were not and are not up there in heaven. (John 3:13, Hebrews 11:13)