Do you want to know why battlefield games are getting boring and stale? It is because we are not getting anything new, every game release is always the same; Same limited class customisation mechanics, same gamemodes all over again.
I do have ideas that I'd like to share that might be used for the next game.
Lets start with the basic stuff first.
Ideas that apply to the general game
Inventory system instead of classes and slots.
One of the thing in my opinion that makes the game so stale and boring is because of the limited customisation. Instead of this 4 class choice and "Ability" slots, I suggest to implent an inventory system. You will not be so restricted by the limited choices, you will simply have the option to insert available items (Grenades, Medical items, etc.) and weapons (Handgun weaponary, assault rifles, Anti-tank weaponary) into your inventory. You can put whatever you want into your inventory, as long as it fits into the limited space.
Also, the more items you put into the inventory, the slower your character will move, and more limited sprint speed.
Sprinting and Bunny hopping.
I'm certainly do not ask for "Realism", I do just think sprinting and jumping needs to be limited, to limit this CoD style run-n-gun playstyle which In my opinion does not fit battlefield. I'm sure this would allow more tactical, refined and authetic gameplay rather than being another Call of Duty but with bigger maps.
Also, quickscoping and shotgun bunny hopping certainly needs to be limited and fixed, being 360 quickscoped from close range by random CoD vet is very annoying.
Character progression tree
I do think that this will definitely flesh out the game. Progressing through the skill tree should unlock you more abilites. From basic starting stuff like medic or support skills to better stamina and hardening, weapon and injury handling abilities
You rank up by collecting XP, therefore more skill tree score you earn to unlock and learn new skills for your character.
Weapons do not need any "Upgrades" like how it was in Battlefield 5, Battlefield 4 weapon customisation is perfect already in my opinion, though some few additions might be needed to give more freedom.
Ideas that apply only to Hardcore mode of the game.
While it is true that in reality you dont see any interface stuff in your eyes, but not being able to see your health is like not being able to feel pain when you are hit or injured, which does not add anything to the immersion and just makes the gameplay tedious. Everything else is fine.
Being injured (Like having only 45% of your health) should slightly reduce your accuracy and in terms of animations make your gun handling a little bit shaking. Also, being hit should stun you for a mile second (Not in terms of movement, but in terms of aiming and shooting) and also push you for a few centimeters, to actualy give this weight of feeling being hit by a gunshot. With also this the victor will not be the one who has weapon with better DPS (When both start shooting at each other), but rather, situational awarenes, reflexes, aiming abilities will require to become the victor in engagements.