Because fixing the Wild Hunt and Eredin and Radovid and all the politics is far harder than adding some banter with Triss. Impossible without a massive overhaul of cut scenes, story, gameplay etc. But this thing with Triss. There were other topics that could have enhanced her character arc - the plotting of the lodge, her role in TW2, her indecisiveness in TW1...and they went with romance banter.
They did not do those instead of just adding some banter for the same reason other things you listed were not changed - the banter was much cheaper to implement. For each of the three main NPCs, patch 1.10 added only about 2-3% more lines of dialogue. The main problems with Triss content in TW3 were caused by some changes and cuts late in the development of the game compared to how it was planned in 2013, but this is not something that can be "fixed" by adding a few lines of dialogue. In the Skellige stage of Act 3, she would have had more screen time (and was for example planned at some point to appear in The Sunstone quest, as well as some deleted content), but in the released version of the game it is only 10 lines, some of which are not actually used. But this is a part of a general problem with that section of the game being rushed and cut down. Also, the "Brothers in Arms" quests were planned to be part of the main quest in Act 2, and this includes Now or Never in particular. Additionally, the quest Carnal Sins would have been before Now or Never and included Triss in some role (cut in the final version of the game). However, all those quests ended up being side-quests only, with their recommended levels set in a way that suggests playing them in Act 1, and Now or Never (level 14) before even going to Skellige (level 16). For those who romance Triss, playing the quests in this order results in a broken narrative, a long section of the game having no content, and no consequences to their choice (as the Skellige quest line and Ugly Baby were not designed to be played after Now or Never).
What would really have been needed is more quest content, but the patch only ever promised to implement "tweaks".
The big things left (to me) are
(1) Main narrative 3rd Act/Wild Hunt - would have to create multiple quests and probably new areas to properly portray the wild hunt.
(2) Crafting/alchemy - whole system would have to be re-balanced and organized.
(3) Lack of characters - adding in old characters (i.e Saskia and Iorveth) would require making multiple quests to make it more than cameo appearances (which would be unsatisfying anyways).
(4) Post ending play - many lines of dialogue, change npcs in multiple areas depending on game choices (i.e. changing soldiers in certain areas).
Some of those may not actually be seen as problems by the developers themselves:
(3) I do not think Saskia was ever seriously planned to appear in the game, and while Iorveth was until 2014, it is not clear if he was cut for resource or story reasons (they did add Ves in his place instead). In the latter case, I doubt CDPR would want to add him back even if they made an enhanced edition of the game. It is not possible to include every old character from the previous games and books, and some just make more sense for the location and story of the game than others. But it does look like some cuts were made for lack of time or resources, for example, Sile (if alive from TW2 import) was planned to appear in Battle Preparations and at the final battle (like above, the second half of Act 3 seems to suffer the most from cuts), but ended up being reduced to a cameo role of dying in the prison.
(4) According to comments from the developers, there are no plans to implement a post ending world state with the main characters. It would require a lot of work to do it properly, and they do not want to add it only partly implemented as a kind of fan service. In any case, the game has to end at some point, there can only be finite content implemented for the characters (barring the addition of something like a Skyrim style radiant quest system), so it might just as well end when the credits roll. The post-ending play is really only a bonus feature, and was not intended to be part of the story. But maybe the epilogues could be extended somewhat - they were also cut down in the development, although it may have been for "artistic" reasons. The dialogues in the inn in the empress ending do seem to be rushed.
As for the others:
(2) Gameplay tweaks are being made all the time in the patches, and there will probably be more until the development of the game is discontinued at the end of 2016. Ultimately, it is quite hard to make the game perfectly balanced in a way that satisfies everyone. I personally still find TW3 an improvement in this aspect over its prequels (which is not to say they are bad), however, and is also better than popular competitors like Bethesda's games.
(1) The last act of the game could definitely be improved with some additional content, and I also find the second one already lacking somewhat, but it is quite possible the Wild Hunt was not intended to be portrayed as complex characters in the first place. That is why they all have
only minimal amounts of dialogue. I doubt CDPR would have considered cutting them down that much if they were considered important - chances are those dumb one-liners are about all that was planned. But there would have been bigger and more spectacular battles, such as the Wild Hunt attacking Novigrad - we got the less ambitious On Thin Ice quest in the place of that. Also, the fundamental structure of the main quest line was not changed much since 2013 (see
this post), other than the already mentioned removal of main quests from Act 2, and some cuts in Act 3.
So, in case an enhanced edition of The Witcher 3 was made (despite CDPR saying otherwise earlier), I guess it would be similar to that of the previous game, which added 2 new quests (one each for Iorveth's and Roche's path) in chapter 3 and a bunch of new/improved cutscenes, in addition to some polishing of technical aspects, bugs, and gameplay. I suspect it would not make significant changes to the narrative, make characters much different from how they are already portrayed, or add major new characters.