First off, kuddos to everyone participating on this thread and those who are following. I am pleasantly surprised at how much attention and participation it is drawing. For newer guests, the critique is simple: Triss, romance or not, is inconsistent as an important supporting character she was made out to be by CDPR.
Everyone has different ideas on how much Triss material they would like to see, but it all comes down to dissatisfaction with the current state of her character and interactions.
As romance the demands are clear: her romantic involvement with Geralt should feel genuine and be communicated to the player after the Now or Never quest, i.e. during the KM sequences and third act of the game. As CDPR presented romance options to the players, therefore players should feel that choice matters through in-game interactions. The consensus here is that's not the case due to pacing and narrative inconsistencies.
If not romanced, she should still feel relevant to the story because the developers included her in the later segments of the game to begin with. Every minor character that appears in the final act is given dialogue and interactive moments, while Triss was sidelined as a background piece. She has appeared as an important member of the main cast in previous games by the devs and this should have been displayed in Wild Hunt to show continuity. The argument here is the game should reflect what it is suggesting: Triss is a close member of Geralt's inner circle.
Now...this does not mean that the issues with interpersonal connection is limited to Triss alone. While this thread is focused and dedicated to her, numerous posters have noted that interpersonal relationships and the family theme the devs made central to the story was not displayed between the main cast, and especially when examining Triss. She stands as the figure who is visibly excluded from the main cast, while simultaneously being elevated as important to the Ciri-Geralt-Yen relationship through NPC commentary. This is not conducive to Triss convincingly being important or relevant to Geralt or the player, if the Wild Hunt is considered alone.
The solutions presented so far range from an expansion that incorporates Triss as a supporting character, and reflects the romance choice, to DLC that focuses on Triss. If she is not romanced, dialogue, banter, and any interactive moments should appropriately reflect in-game decisions and remain consistent to her character.
Whatever the remedy, it should not: 1) depreciate the content and characterization of other characters in the current game; 2) be a "patch" that corrects the narrative inconsistencies of Triss' material alone; and 3) take away from further lore or story expansion in the greater Witcher universe. An expansion that provides Triss content for both those who romanced and did not romance her, as well as focus on Geralt's other relationships and build the Withcer universe overall, would be satisfying to see from CDPR.
On the topic of Kovir, I think it is the best location for an expansion because it has the most relevance to in-game events. It holds the best potential for Triss-specific content, whether the romance option was pursued or not.