Lots of Deborah Harry
Kraftwerk in general
Aldo Nova--Cars
ELO-Twilight
Fripp, as mentioned
Many of the above mentioned 90's tunes are, in my way of thinking, an evolution of the above artists. To pin down specific tracks is a bit presumptious on my part, as I am not thoroughly educated here, but surely "Rapture" and "Heart of Glass" must fit in there somewhere.
The varying trance styles today can be traced back to different influential tracks for certain, I am unable to list these comprehensively...
But a quick listen to Roxy Music's first record will be interesting to those with a keen ear for Delerium type trance, as would a listen to ELO's record 'Time" for those who wonder where PVD's thick sounding harmonies and strict 4/4 beats might have found a few seedlings.
Jean Michael Jarre undoubtedly influenced trance possibly more than anyone else ever did. He brought to the mainstream arena the true depths of sonic hypnotism that is possible with minimal chordal movement.
Consider Oxygene...part six to be exact...although the entire soundscape is highly provocative to the trance student. Especially those who tend to worship Sasha.
The warbling of rythmic transients is more a result of filtered disco than anything else. Initially these warblings were incidental, but mixers began to discover an intrinsic coolness to the non-exactitude of a single transient, preferring the actual 'drum' to be anything but.
If you take a close listen to DJ Dan's early mixing he actually approaches certain trance elements in an incidental way. The correlation is not at once apparent, but its there.
If we really wanted to get to the actual roots of trance, we might have to go as far back as Claude Debussy, who was the first musician to successfully render appealing sonic landscapes with repetitive arpeggiating, both tonal and atonal.
A listen to Claire de Lune is a good place to start, but an in depth study of his music is worth any effort it requires.
I do not consider the tenth century monks chants to be the same type of influence that say, Debussy was. The reason is that the sound is a finished sound as it stood, and the only thing anyone has ever done with these in a modern setting is add drums.
The above refernces do not take into account the very relevant impact that African tribal rhythms had on todays dance music, which is an area that must be fully examined in order to comprehend just how significant that influence truly was, and is.