Wow, for an Ambient song, this is great!
You're seriously really good at making Ambient. This is really calm and easy to listen to, and it instantly puts you at ease, and those are the main factors that every Ambient song should have. I really can't find many defects in this, great job!
The melodies are really nice, especially the pad's melodies that come in the middle of the song. They're just so relaxing, and even when you press the 'Mute' button, you can still hear those amazing melodies boom around your head. Maybe along with that instrument at the beginning there could be another instrument like some delayed plucked instrument which plays a melody to accompany it, but it's not too much of a problem. What I'd wish for in this song would be more alternate melodies for sure. The pad in the middle played a beautiful melody, but sadly it started to get annoying after the fourth repetition. You should vary the notes you play a little so that they don't get so predictable to listen to throughout the song. Even though you said you'd be adding vocals to the song, it isn't really an excuse to not vary the pad's melodies at all XD. On the other hand, the high bells' melodies were a nice addition to the song: I wish I saw more bells' melodies around the song though. Sorry for the long paragraph.....
When it comes to the instruments, they're really impressive. I liked the pad at around 1:08 most, but all of the instruments sounded nice. I strongly suggest adding some piano with a delay on it, to give it that mystical and majestic feel. Also, one thing that I really think you should add to the song is more sound effects. I heard some thunder sound effects, which were a nice touch, but other than those, there were no sound effects. Some space-y sound effects, maybe some windchimes or more weather sound effects. Some things that could remind you of distant memories such as kids laughing and playing a game or something like that. Use your imagination, and spice up the ambient song to make it both relaxing and fun to listen to ;).
I can't complain about the transitions, they were all great. I liked the transition a 1:08, it actually managed to switch to that slow pad really well. TBH I'd never manage to do something like that without messing the whole transition up xP. Maybe the addition of sound effects and stuff like delayed reverse cymbals and the like could make some transitions sound better, but that's just a small suggestion. New instruments were introduced well, too, and I liked that small buildup of the main instrument at the beginning.
The song's structure is pretty weird. First, you play that instrument at the beginning, then move on to a pad, and then both the pad and the instrument at the beginning. It actually works pretty well in this song. As for the variety, as I said above, this needs more alternate melodies. If you ever add those vocals, it would add more variety, but I'd suggest adding some more counter-melodies to this song so you could vary practically everything ;). The intro was really good, with a nice buildup, and the outro was perfect. I don't usually like fade-out endings, since they leave the listener wondering why it ended like that, but this fade-out ending fit really well with the song, and sort of symbolized how memories fade away. I liked that you added that thunder sound effect at the end too.
I'd comment about your drums, but you didn't have any :P. Well, that is, except for the part at around 0:40, and even then they were pretty quiet. I know you were going for a mystical feel, but drums can be made mystical too. Reverbed kicks, heavily reverbed shakers, delayed reverse cymbals, dance claps, and maybe add a whole new style by using congas, bongoes and the like. I think the addition of some percussion would really do wonders to this track ;).
Overall, I really enjoyed this (heh, now you know what it feels like getting long reviews xP). I'd give it a 9, but you get a 10 because you managed to make me download the genre I dislike the most, and it's Christmas time after all XD.
10/10
5/5
Download.
-Review Request Club-