The only explanation I can really get for Sand Troll customization is that they aren’t Sand Trolls, but instead just Darkspear Trolls who have that skin tone. Sand Trolls are apparently related to Jungle Trolls, which is why that is the only logical explanation. Especially since any known Sand Troll tribes hate the Horde and we haven’t seen any new ones appear, so it just makes little sense that we suddenly see them in the Horde without any explanation. Moreso when something like Forest Trolls, who were a big part of the old Horde during the RTS days, should have more priority and significance over random enemy tribe that would somehow join a faction they despise with no in-game explanation.
But yes, it is really weird to see something like Bronzebeard and Dark Iron Dwarves as their own race, and something like Wildhammer Dwarves, who could easily be their own unique race and has had importance to the Alliance since the RTS days (they have a statue of a leader in front of Stormwind) gets to have Bronzebeard Dwarves LARPing as them. It really muddies the waters when it comes to playable races.
People have wanted a sub race system since Cataclysm, often citing Dark Iron Dwarves and Mag’har Orcs for such a system. Maybe coding it with the spaghetti Frankenstein coding that is WoW is too difficult to implement or maybe the allied race system was something they wanted to market for an expansion. Who knows?
It is a reason I’m vocal about the whole customization race thing. Compared to actual races, customization races just feel empty and really limiting as they are. Moreso when there are issues with Dark Ranger customizations. I made an entire post on the subject a long time ago talking about concerns of it and trying to find a reasonable solution to customization races having their own feel in gameplay and identity. It’s difficult to find that balance to say the least.
Frankly, I worry about some races becoming customization races because of how lackluster they seem to have been treated. For example, if we ever saw Kelfin as a customization race for Goblins, I worry that racials wouldn’t really be changed at all, so we’d end up with a race of aquatic beings who live in the deep ocean and have gills (hence the name GILgoblin) being able to drown in a matter of seconds in a shallow pond.
It’s a reason why for something like San’layn, I’m especially vocal. If they’re playable in some fashion, I want them to have a presence in the game in storylines. I want them to have a vampire theme and features. I don’t want them to be LARPing Blood Elves who just suddenly appear without any explanation and don’t have any presence in the game.
Player characters are the most important part of a MMORPG to me, and I want to see any race that becomes playable in any fashion given the love and care to make sure they are part of their respective faction. Not just a parent race cosplaying as one. Certainly, something like Wildhammer Dwarves deserve much better, as would Dark Rangers and any other customization race.
Sorry to be all grumble grumble on this subject lately, but I still feel it’s something important to say. Customization races can work, but they need better treatment than they are currently getting. And I worry for something like San’layn, that what makes them awesome and the potential they have will just get lost if slapped onto Blood Elves and any other race that would get San’layn features.
Just how I honestly feel is all. 
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