Here's some of my minor niggles. Completed everything on my main character (Titan) but didn't want to grind out Forsaken on the other 2. However, I can access all Strikes, NFs, Gambit, etc on the Warlock and Hunter. Picked up quests for Malfeasance and Thorn on the Warlock, completed steps as and when. Completed Thorn PvP requirements and had Chasm of Screams in pursuits. Malfeasance quest in pursuits at Corrupted mission. So a friend asks me if I will help with Corrupted and Chasm of Screams. Go to my Warlock... no Chasm strike or quest step. Corrupted mission active so I join his fireteam... but I'm blocked because I "don't have access to this activity" because I haven't completed Forsaken on this character. So a Strike I could have done is gone, but the one in my pursuits is inaccessible. Ended up just doing it all again on the Titan. On another point, I got Wishender months ago on the Titan. Would like to do Shattered Throne on another character, but can't join friends' fireteams, again because I haven't completed Forsaken campaign on the other two. Also can't play heroic versions of the next missions on the milestones for the Warlock and Hunter because I haven't played them on normal.

Pinnacle quests are account wide. Exotic quests are per guardian. Some of them can be abandoned, some cannot. Some only show up at their respective vendors but will not be stuck in your inventory unless you pick them up. Once you complete an exotic quest, you can purchase additional exotics from your collection for your other guardians. No need to complete exotic quests 3 times each (unless you want to). D1 had a terminal in the tower where you could go to pick up abandoned quests. I'm sure Bungie is working on lots of other stuff with all the changes (Free to play, switch to Steam, cross-save, ShadowKeep, etc). This is probably pretty low on their QoL list. If something this minor bugs you so badly, you may need some professional help.

Here`s an idea If you want to make players grind out lets say an exotic weapon quest on an alt then maybe have a few quests where the weapon is character specific and can only be equipped on that character I`m sure they did this in D1 with an exotic sword, I may be wrong as I`m getting old, but if not maybe it`s something you should look at Bungie And as a second thought it wouldnt take that much work design a weapon with traits for that class and call it something like Warlocks Bane, reskin it for a different class giving it perks specific to that class and call it i dont know Titan storm , then rinse repeat for the other class

They are already doing this. Black armory vs Menagerie.

Didn't they do that with menagerie? I know it wasn't that way with Forges and Reckoning, but they made the change for Opulence.