The game isn't currently very alt friendly, but lets have some fun. Pick one game feature that you would like to see as an account wide function. It doesn't have to be a current game feature, it could be something that was taken away or something you've made up. For me - achievements. (Achievement rewards, gear, fluff, etc., could be unlocked through achievement merchant).
There are already things that are Account Wide. For instance, the rewards for helping Yun Zi The Traveling Panda. I had one character do his traveling quests (which are so easy with Google) and ALL of my 19 Starfire Server characters can now go get Pas Yu Carrier of Things Rewards. I think there are other instances of this as well. But this is the one that I really like. Most of my characters can wear the Chain Mail Armor of Tol'Ren that Pas Yu "sells" for free.
Yeah, the Panda is really good and I look forward to this Summer's goodies That also shows they have developed the ability for account wide unlocks to a certain degree.
More flying mounts at level 30. The raptor jumps, and there is a flying mount for argh of course I don't remember the names. The lava/flame god and I think for Frostfell. I can get a flying mount with the expac, but that is just the character that jumps to level 120. ( Or 110 in the past. ) Leaves my other toons out. A slower flying mount for lower level toons that come with an expac. The faster one goes to the 120 toon.
Adornment unlocks and achievements is something I have been wanting for alts for a looong time. Especially with how much power they put on adornments the last couple of expansions.
I want to step through a portal into a room where I select the character that I want to be. Then step into another portal to enter the Realm of Everquest. If I die in the realm, then I end up back in the character select room. Oh, I can also spend an unlimited amount of time in the realm as the avatar that I chose, and when I step back out, no time passed in real life. Why not wish for the better thing when it has the same chance of getting done as shared achievements.
Mount / Merc / Familiar training, should be if your main has it maxed then if a alt has same named familiar etc should be able to instantly max out. i've been tempted to try out lots of characters now deffo with the revamp but cant, due to i know they will be very ****.