The song 'I Can Only Imagine' was inspired by the lead singer Bart Millard's relationship with his father. His father was abusive in his early years but later had a transformation. Bart wrote the song as a way to imagine what it would be like to see his father in heaven, free from the pain and difficulties of his earthly life.
Well, it could be about a person's wildest dreams or fantasies. Maybe it's a story where the narrator can only imagine certain events or situations because they are so extraordinary or out of reach in real life.
It could be about a variety of characters depending on the context. Maybe it's about a person facing a challenging life situation or a character on a journey of self-discovery.