This sort of thing exists in FFXIV, it existed in SWG as well but to a lesser extent. While players will never and should never be able to alter the actual game play and mechanics, the ability for them to design and share cosmetic items such as armor skins, weapon skins, stronghold furniture, character cosmetics, etc would be great. Some enterprising individuals could even potentially change the visuals and overall art style of the game if they put in the time and effort.
Personally, it was always amusing to me in SWG to have a Sarlacc Pit following me around everywhere instead of a Probe droid. It was fun to be driving around in a Snow speeder instead of an X-38 land speeder. I'm the only one that saw it because it was client side only and it gave me no advantage what so ever, in fact sometimes it proved to be a personal inconvenience. Basically anything that doesn't give anyone a game play advantage over anyone else? I'd say go for it.