I agree that mod support would be great, but if it requires an entire separate steam workshop API to be built from scratch, that might detract a good chunk of the budget that might be better used to making the game the best, most polished it could be, with features the community are asking for that might exist outside of the original plans.
That being said, games of this sort always end up having a large and thriving modding community, and lots of people only even play the games because they can mod it, or because they want to mod. And games with a large modding community tend to maintain longer levels of interest than they might otherwise (see: every game over 5 years old with giant modding communities ever, as well as some of the newer games, like SDV and others.)
However, ultimately, tbh, I don't know anything about video game programming, only some web code, so I have no idea how difficult or involved making this sort of interface you're suggesting would be or how much budget it would require.