I don't exactly know the right answer to this question. In a lot of ways I like that the card pool is somewhat small and manageable. Small changes, card adds, etc. can alter the game quite a bit. Sabbath added a whole new deck type, the few changes that were made in the swap update completely changed most of the decks that were played. Some cards are completely useless because there are cards that are just flat out better in pretty much all circumstances. Those should be reworked. I think that if most cards had some interactivity it would add a lot to the game. Maybe they do something to your own cards on the board or the other side of the board, but that would at least move the game toward the direction of each play mattering more. I think all games suffer from the lack of deck variability because ultimately a few decks will prevail. RNG is not, in my opinion, the solution to this problem. I think most people would prefer more strategic element rather than just plain luck.
After all, chess never changes, has been around for hundreds of years and is still thoroughly enjoyed, very competitive and ultra strategic. People play it for years and years, far more than they will stick with most card games. Maybe if cards were more similarly powered (golds to golds, silver to silver, etc.) but they had multiple abilities to choose from so there worth would be tied to when and how they were played relative to the state of the board....just tossing out an idea.
This is a good topic though and it would be great to hear some specific ideas that people have that could make the game even better.
I really like Gwent though and have more fun playing it than other CCGs.