Sorry, i meant a costume set, not an armor set.
Half-off elevator costs would have been nice, but wouldn't make people with elevator passes happy.
Hm... I guess you are right about the new content thing, they would just re-use it ever year and would get boring.
Still, would have really liked a costume set, and not one birthday themed, just something cool.
The wings and Rose costumes were nice, but they were costumes and accessories; as are the cake costumes we had for the first anniversary. I agree other games have significant events for their first anniversaries, but what else could they do? They could have done half-off elevator charges for a couple weeks but that would hump the economy. If they reintroduced those old frivolities their prices would diminish due to greater availability.
If they make an armor, not a costume but an actual armor, only available during the anniversary it would push players to obtain and sell it for ridiculous prices, regardless if its stats are worth using. They were probably scrambling for what to do for the anniversary and decided to go with cake costume helmets. Any new content only available for the anniversary would inevitably be reimplemented every anniversary or become a permanent addition, contradicting with the point of the special annual event. For example, say they made a minigame specifically for the anniversary; people would want it all year round, others would hate it for whatever reason. Would they want to deal with that?