I think it would be rad to have Collection Lists be account wide, perhaps per armor class. If I put a transmog together in Collections to go farm up from old raids on, say, my Alliance mage, I would like to be able to see that same Collection List on my Horde Mage (and preferably also all my priests/warlocks because they too can generally farm the same pieces of gear to assist in completing the transmog).

This can already be done. The collections list are linked to profiles in the options menu, so you can create a custom profile and then share it with the characters you want to farm with. The setting can be found in the options menu -> List Profiles tab -> Collection List