The Collector Edition of World of Warcraft: Shadowlands offers some interesting things. In addition to the Ensorcelled Everwyrm flying mount, you'll get Vestments of the Eternal Traveler and Illusion: Wraithchill by puchasing Heroic or Epic Edition.
Vestments of the Eternal Traveler
This transmog set is included in the Heroic Edition ($59.99) and the Epic Edition ($79.99). It is available for Horde AND Alliance players.
To obtain it, you must complete the quest The Eternal Traveller obtainable by invoking the Ensorcelled Everwyrm flying mount. Simply click on the little orb on its back to bring up Xolartios who will offer you your quest. There is only one thing left for you to do: farm the living beings of Azeroth until you get 40 Echoes of mortality!
Illusion: Wraithchill
Illusion: Wraithchill is included in the Epic Edition ($79.99). It is available for Horde AND Alliance players and gives your weapon a spectral hue.