You can change the flatness by making the ground a displacement. Doing that will add variation in height and make the map feel unique. Maps like Upward do a good job on this. You can make the ground only a few inches taller or shorter and people would actually think they're walking on mounds of snow. Here's an article on how to make displacements:
https://developer.valvesoftware.com/wiki/Displacement
You can change the sightlines by adding prop walls or actual walls that can give people cover from snipers. Barricades or what not.
Also, try making the cart go through tunnels of snow, igloos, snow canyons. If you add variation in where people are at a specific part, it can make the players feel like they're in different places. Add a lodge with tables, fireplaces, and other stuff. Add a place where kids might have had snowball wars in. Make the map feel like you're in a snowy area. Think of a ski resort. The snow theme is an interesting choice. You can add more and make it the map you would like to play in the game.
And add extra pathways. Use the clipping tool to add hallways. Use it to add small spots where spies can decloak. Add atleast 2 routes to attack someone from. If you don't know how to use the clipping tool, here's another article on how:
https://developer.valvesoftware.com/wiki/Clipping_tool
If you want good feedback, here's good feedback. I love the snow theme, I like the christmas tree there, although it could use decoration, and the map is an interesting idea.
Please take this stuff and try to find how you can do it. And it's not the fact that everyone hates it. It's that you don't take criticism and they're telling you how to NOT make everyone hate it, but you keep being in denial. If you have to, take a week-long break and come back to your map. See what you did wrong, and change it. Or start over if you have to.