Add one thing, take one thing.

Add: character backgrounds as origins: Start the game as a templar or a warden etc
Take away: Race options

Add: Scantily clad male desire demons. For uh, reasons.
Remove: Scantily clad female desire demons. Also for reasons.

Myself I say remove Elfroot (what a pain to gather) and add porkchop Oops sorry porkchop is what I name my malbari

True some games don't really utilize it well. I always think of Deus Ex that does handle that mechanic really well, so I want it to that extent. 

Add: More reactivity. I would like more reactivity to class, race, major decisions, etc. I would also like to have quests include dialogue options and solutions based on class so that it feels different to play different classes outside of combat as well as in combat.

Remove: Multiplayer. Leave multiplayer to the Austin studio. Edmonton could return to making high-quality single-player-only games.

Add: Qunari as playable in DA4
Remove: Dwarves as playable in DA4

Remove: all magic-like abilities for non-mages, like the earthquake ground slam and poof I'm invisible "stealth"
Add: more "mundane" combat skills and moves for warriors and rogues as well as stealth based on character skill and ambient light levels

Add: Dragon Age Inquisition Character Creator, with all the options and customization depth plus the mirror. 

Remove: crafting. For me it added little to the game but repetitive annoying grinding. Just make more unique items to loot. 

That reminds me:

Add: Unique specialisation tree for each party member, based on their stated in-universe talents ala DA2.

Remove: That guard/barrier nonsense in DAI. It's just an extra HP bar designed to drag shit out.

I cannot believe someone else sees this. Seems to be a pretty unpopular opinion on this board, at least to my experience. Sure, the Chantry annoyed me even in DAO, but it did not feel as pushed on a meta-level and we could be mean to arrogant priests. DAI and content since then went the wrong way. Sometimes I fret that DA will take the same poor direction as some ME content, that is everything being about "normal" (i.e. andrastian here) humans doing awesome stuff (tm), while everyone else is either at bit too comically evil, stupid or both. Case in point: portrayal of Dalish elves since Masked Empire.

Restore the scope and flexibility of customisation of skills, spells and abilites that we had in DAO. Let a warrior learn dual wielding if they put enough points in dexterity and a rogue wield certain swords and axes as a dual wielder provided they have enough points in strength. Let both classes be able to at least use a crossbow, which needs less skill than a long bow, but be able to become a skilled archer if they wish by sacrificing some other area of expertise. Also be able to switch from ranged weapons to close combat weapons as the enemy closes on you rather than being stuck with one or the other when it makes no sense to do so. Have a greater range of spells to choose from. (All these were options in DAO). Go back to having more than one specialisation for the PC and a choice of more than three available to them. Go back to giving the companions their own unique specialisations. (Something that was good about DA2)

Take out collections and giving awards for completing them: bottles, herbs or whatever. Also take out having a golden disc that lights up for important objects but also useless ones (like bottles). I may have difficulty in finding something important but that will just make me have to look harder and at least I won't be constantly swearing at being told about something I really don't want or need.

Take out making us collect something across the breadth of the game (shards) in order to access somewhere that has absolutely no relevance to the main plot, has nothing of value at the end of the quest, a boss fight that is ridiculously easy because you have leveled up so much by then, and a mural that is never explained in that particular game. To make matters worse it is never explained how the objects you had to collect came into existence in the first place, how they came to be littered around across modern buildings and land features and yet have a connection with opening a complex that dates back to ancient times. To cap it all, in order to find them you are forced to look through an object that was created in the most horrific way and to which you have moral objections to using but keep reminding you of their presence with a ringing noise every time you go near them.