Guild Wars 2

Even though i can see the fun and benefit of what you are suggesting, i personally don’t think it would work all to great considering with the limited storage space people have with banks being shared among characters.
Achievements alone should be personal to a character unless it is a specific account based achievement. Like “This player PERTICIPATED during 2012 Halloween” etc etc…

What would be a smarter system would simply just to have items you’ve gained through holiday event’s to be account bound. That way you can interchange the gear among any character… Although you would have issue with type of gear (light, medium heavy) and so on…
It would still be a far better solution to your idea.

Another idea, even though slightly less ingenious, would to simply have a specific NPC somewhere that would duplicate any item you wish for a small cost.
Depending on the item’s rarity or so on, the cost would change.
However you would again have issue with the specific light, medium and heavy gear’s seeing as it only duplicates that exact item and does not change them up to another gear type. And quite frankly i can see this being a abused system as well.

Overall, i like your idea, it has it’s merit but i can see the logistically problems with it.