OK, great question... I'll tackle this from a chronological perspective re Enya's albums and pick songs that I think could have had amazing (Lyric) videos:
The Celts - AldebaranI know there is a video out there of this song, with Enya playing piano and singing into a microphone sitting in a semi darkness. However, in this day and age, I think this could have an amazing lyric video with some amazing drawn artwork, or things like stars shooting through space etc.
Watermark - Cursum PerficioProbably a real long shot, this one, but this would have something to do with Marilyn Monroe's grave, and then proceed in a sort of dark way - or vice versa, it being a dark video and ending with her grave instead. In fact, the latter appeals to me more as it coincides with the "My Journey Ends Here" idea.
Shepherd Moons - Marble Halls (this was a tough call, as I feel Angeles and Afer Ventus could have splendid videos too, but this one is iconic - I'm not aware if there is a proper video of it)
I'd have this one set in a ruin of an old castle somewhere, and Enya sitting and walking around, off set with scenes of Castle life before it became a ruin, but then abit more fairytale like so it fits the lyrics. Enya wearing an amazing dress as usual
The Memory of Trees - China RosesWith such vivid imagery, how could this not have been turned into a video! Flowers, porcelain trinkets, a story of love and dreams intertwining. Enya in a gold coloured dress, moving from pictures to items on cupboards or tables to doors leading into a magnificently blooming garden with spectacular colours.
A Day Without Rain - One By One(I could have gone with Flora's Secret, but imho that would turn out a bit like the CR video I described above, and let's not repeat ourselves. Plus, I think ObO has more of a narrative)
Like the Only Time video, this video depicts seasons - but in this we see a singular tree, and two lovers, moving to and fro around it, and to and from eachother. As the seasons progress, the story gets told, and at the end, we see Enya appear (as a cameo almost) to finish the song.
Amarantine - SumiregusaThis is my preferred choice - Think of a Japanese / Asian painting, done in watercolours, and we see an Eagle flying above a majestic, colourful landscape, rising and falling. As the song progresses we are drawn more into a 3D world, with watercolour mountains passing by, the Eagle flying over valleys with cherry trees and typical medieval/feudal Asian shacks/buildings. We swerve and sway with the bird, the camera changing angles again and again, and with every new visage more and more colour comes in, until, at the end, we zoom out, away from the eagle, to find a watercolour painting of a wild iris coming into focus, hanging on a wall....
And Winter Came - White is in the Winter NightHow could I not choose this one - the most Christmassy song she's ever made, with a boisterous pace but so much fun and joy. I imagine children playing, mistletoe, lit and decorated christmas trees in the open, snow on the ground, stars in the sky... this is one of the most imaginative songs to have a video for.
Dark Sky Island - Dark Sky IslandI think most of you know that this is my favourite song of the album, and a bit like One by One, it has such a delicate narrative but so clear. The ocean would be the centerpiece here, with a shoreline... perhaps full of light, then again dark, storms at sea, calm skies...until, at the end, the glimmer of the sunrise.
PS: Other choices for DSI would include Sancta Maria, Astra et Luna and maybe also Forge of the Angels.