Most of my thoughts come from years and years of playing destiny and seeing the way the game has changed over the years. They can be broken into two categories for now. Buildcrafting and general gameplay
BUILDCRAFTING:
This is definitely my favorite aspect of the game. Even though you killed permanently my warlock combat medic build.
To start things off the seasonal artifact mod needs to change drastically. With every season you push 1 or 2 subclasses and a small selection of weapons with extremely good seasonal mods but the other subclasses/weapon types see little to no use because of it. Expand the mods for a more broad application to any subclass.
(FIX)
An example would be the current seasonal artifact mod Lightning Strikes Twice. This is arguably one of the better mods that has been given to the player base, but it only benefits the arc subclass. A simple change to allow it to apply to any subclass/grenade would help this one. Rename to "Dont throw one, throw TWO" or something less cheesy, keep the grenade regen and have solar kills extend it for solar, void kills extend it for void, stasis kills extend it for stasis etc. Yes i understand the argument that that would inject some ability power creep into the game, but thats what this mod does anyway, only it forces you to use a subclass you may not enjoy using.
The same can be said for the weapon/champion mods each season. With the rotation of champion mods you can make the argument that it keeps things fresh, but after having been a part of the avid daily playerbase with access to all content in Destiny 2 and now being more in the casual player base due to work, being forced to use weapons that i may not have or enjoy using or am even GOOD at using just to get through content after a long day of work when all you want to do is relax and have fun is well....no fun at all.
(FIX)
Allow us to assign our prefered champion mods
Example:
Assign trace rifle to be anti barrier (cannot be unstoppable or overload)
Assign autorifle to be overload (cannot be barrier or unstoppable)
Assign hand cannons to be unstoppable (cannot be barrier or overload)
GENERAL GAMEPLAY:
Champion system 2.0
Have champions with multiple mods applied to them. An (extreme) example would be an Overstoppabarrier Hive Ogre
Shoot the ogre with overload to stop health regen
Shoot the ogre at the same time with unstoppable to stun
The champion throws up barrier periodically
There will always be an argument between avid hardcore players and the more casual playerbase. I think you guys/gals are onto something great with the idea of opting into harder content in season 21 deep dives. However i think you could go further by not forgetting the casual player who either does not have the time or the friends list to complete certain activities. This includes dungeons, raids, master/grandmaster content and various other difficult to solo content.
I think there should absolutely be certain things only available to people who put in the work to achieve it. Trials of osiris adept weapons are a good example, however grandmaster adepts are not. The casual player does not have access to adepts so i propose this
(FIX)
Institute a currency based system, either physical currency like spoils of conquest or reputation based through vendors.
After you earn enough of whatever Mcguffin you choose to use you have the option of purchasing 1 roll of the current adept weapon. You can further this by allowing a player to spend a large amount of currency to guarantee a perk of their choice on the current adept weapon. An example would be
Spend 25 currency to guarantee a perk in the 1st column
Spend a further 50 to guarantee a perk in the 2nd column
Spend another 100 to guarantee a perk in the 3rd column
Spend another 200 to guarantee a perk in the 4th column
Spend another 150 to guarantee the masterwork on the weapon.
(All numbers are simplified for ease of understanding and can be any number)
Just some of my on the state of some of the gameplay features.