This is what I'd like to see on the next installment:
Rhode's status - I don't like her being a statue and she was blind by her false justice. Now that rivellon was saved by those who she tried to kill. what will happen to her ideals now? Will she realize that what she was doing was wrong and turn over a new leaf? or, will she still make the dragon knight extinct? I hope she at least appears on the next installment as a quest giver or someone of importance or at least a reading material about her

.
Mini-games - A puzzle game or some reflex game or some card game like poker. The game mechanics will be explained in the game of course. At least, there's something that the gamer will put their minds else where. There could be a tournament of some sorts or quest related or a means to get some powerful items. I remember the days of gold saucer of final fantasy seven hehe.
Romance or relationship development - By this suggestion, you could also add some love rivals or love consequences on the end game scenario. I'm fond of this feature. I found this on the two games(witcher/Dragon Age) that are essential for character development. There's also the intimate scene of it but I believe Larian isn't going to implement this scenes. If you remember the bone that transformed into a human, you will notice that he transformed with a brief haha although he did say he was naked xD.
Mind reading - less the xp debt.
Hunting we shall go - I would suggest powerful mobs that require actual tactics. Its usual in my game that I use brute force against them.
More "plays" - I like the quest in the playhouse. You could summon a real demon from acting! haha.
Keys - Don't count them as an inventory. I couldn't find the reason why you should throw an epic armor for a key that use up your inventory.
Battle tower companions - If the game is a story related dragon knight, I want some of my companion as an "extra". You know, for the sake of aesthetic or some non-value added dude that won't make your character stronger. Its like the hairstylist who came out of nowhere. You could get some ideas though from suikoden game

.
Good ending - The ED was awful when it ended bad. It was 5 months after that I planned to buy DKS and finish it once and for all. It was satisfying enough that FoV tried to evade such ending by putting TWO endings. The player could actually change the fate of his game!
Save game continuation - Carry over the items please

. Although, it will destroy the freshness of the game so ignore this suggestion.
A pre-made story - Self-explanatory. If larian got the resources though I could suggest:
Two starting story line - The ending of FoV has a fork, one where you could destroy the invading forces with bellegar's magic or the other one (I haven't tried the other one yet so I don't know if its a really BAD ending). Naturally, the starting point of your story must have at least two. and,
Carry over saves for story line or some quest related or some additional features - I don't quite get this system yet from witcher or from Dragon age as I haven't finished their sequels yet. What I like to see on this system is the status of those quests you finished from those previous saves to the next game. At least, new gamers who try the next game will be obliged to buy the old one for the sake completion.
Side quest consequences - It is already implemented on DKS. Like the masses commenting on the heroism against the flying fortresses. I'd like for it to stay on the next installment.
Amdusias - MAXXXXOSS!! hahaha
Fresh new start - Play the divine = the people will worship whatever ground you walk on. Play the Dragon Knight = The game could be a cakewalk. Newly recruit = A lot of potential. Of course, there is a great story line from those former two. A pre-made items and stat could cure the OP.
Simeon or philosopher - He did say that after 80 years he will write his memoir.
This list isn't exhaustible. I still have a lot in my mind. I could add some next time I visit this forums.