When I first read about accountwide titles and achievemements I was shocked thet they are going to strip me of all my characters Infos of nearly 8 Years of gaming.
In the meantime there is a huge discussion going on on this topic and I read many many posts and ideas which i no will try put in the following draft:

ACHIEVEMENTS:

Make 2 seperate tabs for achievements

First tab
Character Achievements
Keep everything in this tab the same as it is now.
Meaning only actions of the logged in character should work against the goal of each achievement in this tab.
Character benefits from achievemnts should depend solely on entries in this tab.

Second tab
Account achievements
This tab should include the same list of achievements like the first tab.
You should be able to work incremental with all your characters to gain the achievement in this tab.
Meaning you can collect the monster trophies with different characters to get the achievement
and maybe even add up all your Alliancepoints earned by all of your characters to gain the "Grand Overlord" title.
In that way you might get achievements you didn't have by now, just by logging through your characters.
Obviously it won't make any sense to track the character, that first fulfilled such an achievement
because it hasn't done it alone. So just the time when the achievement was earned, should be tracked.
This way no problem exists, if a character is deleted in the future.
And for the first fill use the oldest date of an achievement that any of the characters has, if applicable.
You should then get the title as soon as you get the achievement in this second tab and
it should be usable by all your characters. But no other benefits should come from an accountwide achievement,
except maybe the dye colors.


TITLES:

You should be able to use the title as soon as you got the achievement as an Account achievement.
But there should be an Indication if you got this achievement on a character too.
I would suggest the Following:

Master Angler You have caught all the fish, but on different characters
Master Angler (1) You have caught all the fish, and any 1 character has done it all for itself, but not the one logged in
Master Angler (1) * You have caught all the fish, and this character has done it all for itself
Master Angler (5) You have caught all the fish, and 5 characters have done it all for themselves, but not the one logged in
Master Angler (12) * You have caught all the fish, and 12 characters have done it all for themselves, including this one

This shown number should in my opinion only include your existing characters and not your deleted ones.
Meaning at this time the maximum would be 18 and if you delete a character with an earned acchievement the number shown
should decrease.


PERSONAL THOUGHTS:
I know that an implementation of accountwide achievements in this way would bring more code and more data,
than the one on the PTS. But for me it would be such a huge QoL improvement, that I gladly would accept it as
the single main feature of the next Chapter (who needs a card game or even companions ?).
On the other hand I really would prefer the status Quo than the implementation of the loss of character info.
After nearly 8 years of tracking the individual development of my characters, I would really be frustrated by that.