Add one thing, take one thing.

This may be controversial but:

REMOVE: Choices that branch the main plot so you only get half the story about important characters.

ADD: Having important character details, particularly about the main antagonist(s), available regardless of branching storylines.

It bugged me in DA2 that you only learn certain information about why Meredith felt as she did if you chose to support her. May be she would only reveal that to an ally but I really don't see why she wouldn't use it to convince a neutral/pro-mage Hawke why they were in the wrong.

Even more annoying was that you only get the memories of Corypheus on the Templar path in DAI. He was pretty poorly developed compared with what they could have done with him but at least on CoJ you got some sort of insight. Not only that but using Calpernia as the vessel made way more sense than Samson and seeing as they were both his chief lieutenants they should really have both been involved the whole way through. It just felt very arbitrary and not very logical that if you chose the mage path you only get to find out about the Templars' lieutenant and vice versa.

I would also include in the remove, making important details part of a DLC instead of the main storyline. They did this with Return to Ostagar, which added a whole different perspective to why Loghain might have turned on Cailan, and then repeated the trick with Trespasser. What made it worse with the latter is that anyone playing on an old console couldn't even play the DLC. A friend of mine had to borrow one of my Inquisitors so they could experience Trespasser on my PC and make their choices regarding Solas and the Inquisition in game rather than simply in the Keep.

Yeah, a bit   !

I don't think it's an issue with the branching storyline but rather with the writing itself... perhaps.

It sounds logical that one should miss details of the story if one hasn't talked to or met the right people. You can't know what the Vergen rebels are up to in The Witcher 2 because, if you made different choices, then you don't get to meet them, which is how it's supposed to be.

Not saying it should be the same for a BW game but it might be something to chew up a bit.

I didn't mean that the information in Trespasser should necessarily have been in the main part of DAI but that the information about Solas should have been held over until the next game, certainly the confrontation with him. Learning more about the ancient elves was fine but having a conversation with him as the potential main antagonist of the next game giving his motivations and plans should not have been confined to a DLC. Also, considering they claimed the main point of Trespasser was finishing the Inquisitor's story, that is something that should definitely have been in the main game. In fact I would rather they had focussed on that aspect of the plot rather than Solas, allowing us more time to present our case why the Inquisition should continue. To be honest there should have been an option to say you were not going to either disband or put it under the control of the Divine. May be that resulted in your being arrested and executed or starting a revolt by the common people who had most benefitted from your activities.

Instead much of the focus was on building up the scenario for the next game and then we had Solas offering the justification for why our organisation was a problem. To give one example, apparently the Inquisition was riddled with enemy agents, which was surely the responsibility of our spy master. May be background checks had been a bit hurried while in the middle of dealing with Corypheus but she'd had two years since then to catch up. In any case, surely checking out recruits for Venatori agents was just as vital when we were fighting Corypheus? So I'd have thought the solution was to get a new, better spy master. Still that is all water under the bridge now but I'd still like the protagonist's story to be wound up in the main game next time round, not off screen or in an DLC that is not playable by all.

Unicorns? Dude, why don't you stick to your JRPG roots for this potential wonder? There is no point adding this to DA4. Not every game can be, nor should every game aim to be, the fulfilment of your "if Disney married Square Enix" dreams. 

Actually unicorns must exist in the DA universe because Dorian compared the Inquisitor to one, as in "as rare as a unicorn"*. It surprised me at the time since there had been no intimation of unicorns in Thedas and you don't use an analogy like that if there is no point of reference to make it understandable to the person in universe. However, the game is full of references that only make sense to a person outside of the universe and are effectively breaking the immersion.

So I agree with Bam Bam the Destroyer:

In fact anything that would not make sense as a jokey reference to the person in universe.  To be honest many of them are lost on me outside the universe, I don't make the connection but they feel incongruous none the less.  As a result a lot of Cole's little monologues had me scratching my head as to how they were relevant to Dragon Age, only to discover that they were not.

Add: Other mythical creatures that we are familiar with but only if there is an adequate explanation why we were not aware of their existence up to now and they do have the appearance and characteristics of the creatures in our legends. May be we could make a foray into the Donarks in search of some vital item/information, where they have already been reported to be "flying chimera and wingless dragons". Or alternatively the Tirashan. I'm afraid Panda that there are still a strange new group of elves there but may be the unicorn could be sacred to them.

* To be honest back in DAO I thought the halla were meant to be the equivalent of unicorns.  They were white, with long twisted horns, very intelligent and sacred to their goddess.   Their horns were also said to be in high demand among the rich because of their rarity and the dangers in acquiring them.    Then we discovered they are simply the result of some weird experiments by a crazy deity and the mystical, ethereal aspect of true unicorns was lost. 

Add: beer and blackjack.

Remove: Gold tinted armour (gold's a useless metal to make armour out of).