First thing i play on almost exclusively Alliance side and have since WoW first started, but i have played through some of the Horde side to see things from their point of view.
I have personally wanted to take some of my Alliance toons I play all the time and defect to the Horde for years. If they every introduced a quest-line or an in-game feature which let you take your toon as is and defect to the opposite faction regardless of how time consuming it was, my Alliance toons would join the Horde right now. Even if i had to level the to max before i could do it.
Because i have always felt that the Alliance (Fascist’s R Us), since all their races are very human looking, except Pandaren or Dracthyr but those races are split and are in both factions. Worgen do not count they are cursed Humans. Also they love to maintain their (Holier Than Thou) attitude.
While the Horde races seem to have banded together for a more strength in numbers situation. Even some of the races that joined the Horde only did so after the Alliance rejected them outright or wouldn’t even speak with them. For example Forsaken and Blood Elves. While the Horde has had a few Warchiefs that have caused problems it has also had good and trustworthy Warchiefs. When the Warchiefs that cause the problems the Alliance goes all out to put a stop to them. However when the Alliance has a Warmongering (Genn Greymane, sometimes Tyrande) member as a racial leader of any of their member races while some may disagree with what that leader does they still leave them I power and let them do basically as they please. Even alot of their dialogue seems to want to make Horde out as exclusively monsters that need to be controlled or eradicated.
Even when they finally did away with the Warchief role as the Hordes main leader and gave them a council similar to the Alliance, the Alliance still has its High King while the Horde now has no main voice leader putting them into a clear advantage.
Even the Majority of lands are some of the least Hospitable places to live on Azeroth, while the Alliance has the more verdant and fertile areas occupied and is unwilling to share these areas, which can in and of itself lead to inevitable conflict. The only real self sufficient Horde races that haven;t had there homes destroyed pre-joining the Horde are Tauren, Highmountain, Zandalari, and Vulpera. All of the other lost the original homes in on way or another before joining into the current Horde that we have in-game.
We all have all the Neutral groups that we Rep-grind for in game that their members consist of races from both the Horde and Alliance races. So who is to say a character from one Faction originally couldn’t have decide that they had enough of fighting peoples that they had begun to view as friends and now want to live with their friends.
While there are inherent difficulties and complications with this every becoming a reality. There is the possibility for it to exist in more Cross-Faction race than just the Pandaren and Dracthyr, instead they could be viewed as the precedent setters for allowing more cross-Faction races.
Due to their cannon histories I see it as more difficult to have a Horde Human, Dwarf, or Gnome. Otherwise taking an Orc, Undead, or Troll to the Alliance. However races like the Night Elves, Tauren and Highmountain coexisted for thousands of years before either joined their current respective factions. Draenei and Orcs before the Legion became to subjugate the Orcs. While Goblins are a Horde race that is only a single Cartel of their race and we still have others that exist as neutral Goblins.
They have also addressed this already to a smaller degree with Elves and Trolls whom have been enemies for just as long but are now Allies inside the Horde. the closest the Alliance gets to this is the Dwarves since adding the Dark Iron to the Alliance, and the subsequent adding of the Void Elves. The latter only due to the fact that they cannot approach the Sunwell without causing problems to both it and the Blood Elves which only resulted in their banishment from Quel’Thalas and no actual conflict until the latter joined the Alliance.
While this has always remain a tenuous and remote possibility a best I would still love for this to become a reality at some-point. Since they are now adding Cross-Faction Guilds maybe this is a closer reality than we think. Personally still holding out a small hope for it in the future.
*Perviously Posted