The biggest problem is that we have characters of all different shapes and sizes from ogres to fae to frogloks. No single chair would fit all characters. This is made even more complicated by the fact that you can customize the size of your character when he is created as well as modify his size larger or smaller with many different items and spells in the game.
It might seem easy to make a character look like they are sitting in a chair, but trying to align them properly so that their feet didn't go through the floor or that they weren't hovering above the chair would be really difficult.
This would be a fun feature to have, but for the amount of time it would take to do it right, you guys would probably rather have guild houses or something like that.
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