I wanted so badly to add this to a support post that I noticed via the dev tracker thread with regard to accidental deletion of the Holographic Shattered Dragon Wings (achievement reward, presumably unused in character inventory). But, this is ultimately the better place for it. 
Consider this my “wall of text alert.”
A summary is at the end.
The basic/main suggestion regarding deletion is this:
When deleting a character, ensure that the inventory, including crafted bags used to hold inventory and equipment slots are all empty, or at least provide a warning with a suitable confirmation system.
This would ensure that players will have to plan their deletions and not be able to do so simply or directly. There will also be fewer regrets due to deleted items and more “gold” on one’s account.
Suggestion: Achievement Rewards Reclaiming
Not to be confused with the achievement milestone rewards/chests.
After first obtaining an item, such as the Holographic Shattered Dragon Wings reward normally given one time by a chest from the achievements system allow players to reclaim items that are no longer equipped or present on any characters or in the bank.
A simple clicking on the chest icon via the achievement could offer a menu to verify the reward (where it’s at, or which character has it) and provide a replacement if it’s not found. This is assuming that something like this is practical and cannot be abused by any of the reward types. If it is process intensive, then it’s use would also have to be limited.
It is also noted that these rewards would no longer be one time rewards stuck on one character but be able to be reissued with this system to new characters if the old character that first claimed the item deletes it. But only one of these rewards can be in use on one character at a time if that was the original design intent.
Alternatively or additionally, allow the player to “unclaim” a reward. Let the player return the reward to it’s original unused state, consequently destroying the item it’s bound to or perhaps use a gem store tool to save the item as well. This would allow easy transfer via the bank to a new character. This would also mean any items unclaimed in this fashion would need to become account bound if they were not so originally.
It should also be noted that my example of the Holographic Shattered Dragon Wings reward should not be confused with The Holographic Dragon Wings, which were a rare item drop during the Dragon Bash event (I hope that becomes an annual event).
~~ Summaries ~~
To sum this all up, the main concepts/ideas for rewards (both events and milestone chests) to take away from my wall of text:
1. Earn it and own it forever.
2. Claim it forevermore for use on:
a. Only one character at a time (ex: Holographic Shattered Dragon Wings).
b. Any and all characters, as many as you wish (ex: Zenith weapon skins).
The main concepts/ideas for character deletion:
1. Deletion must be planned by removing all items from the character to be deleted.
a. No equipment can be worn, includes town clothing. Remove and sell or save in the bank.
b. No items in character inventory, including crafted bags and chests used to hold inventory.
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