Your visual development style is very nice. Those mushrooms look awfully high-res, though. If you want the spots to look like they’re bumping off the mushroom, baking down a normal map might be more appropriate than actually modeling all that stuff out. It would be nice if the particles don’t immediately turn off when you leave an area, but fade out. There are many ways to do this: the easiest is to make a new sprite emitter system within the same particle that fades over time. This way you get the benefit of a ton of dying particles for fast VFX and some fade as well. Also, UE4 has EXTREMELY efficient GPU particles. You can render like tens of thousands of sparks a second, and have them collide with the ground, work with gravity, have realistic drag, etc. You can even let a few of them emit light (not GPU particles, but a few CPU ones wouldn’t hurt). For the snow level, you don’t even need fog, you can just render hundreds of thousands of snow particles and let them pile up if you want them to 
But seriously, you don’t have to go that far. Some simple GPU snow particles that stop moving upon collision and slowly fade away (get smaller) would be a great way to add depth and function to the effects in your environment. Make a max collision of at least 2, and set the bounce values very low: this way your kart wouldn’t hit a snow particle and leave it suspended in mid-air. Because GPU particles are so efficient, you can have hordes of them colliding basically as many times as you want, so, definitely use it to your advantage. It would also be nice to have grass wind in the final product. You can use a simple grass wind function in the material for vertex displacement, or you can make your own using sine waves. A quick and easy way to use weights is to either paint them on or use a masked texture coordinate to render a gradient, so the bottom of the UVs on the grass are tied down to the floor while the rest of it flows in the wind. Not really, but the effect is convincing. Put this on the trees too, to give them a little bit of life. 
Your square rocks have a very uniform size and scale. On one hand it helps to separate rocky walls from the surface you want people to drive on, but then again value and color will do that better than anything else, and at the moment your rock mountains look monotonous and visually tiring. Break it up with different shapes and sizes of rocks. Take a look at Wario’s Gold Mine, especially the Mario Kart 8 update for reference.
The mist effects on the waterfall are very solid. This would be a nice area to have some additive translucency, and let the mist fade over time. You don’t need a lot of mist at every section of the waterfall either: a little at the top and more towards the bottom will sell the idea that water’s breaking better than if everything is uniform. If you want a transparent water shader, keep doing what you’re doing, it’s looking great! But if you want reflections, you’ll have to either set up a scene capture with a 2D render target, or you’ll have to get rid of the translucency and take advantage of SSR and physically based materials (water is metallic, btw). One last word of advice: use UE 4.7. It has much more efficient instancing than 4.6, and with racing games I’m sure you’re finding how wonderful instancing can be. This will just run things a lot smoother than the current engine. Create a copy and migrate your project early. Like, right now, to ensure compatibility and take advantage of it. Otherwise, you’ll be seriously limited by how many meshes you can instance at once. I hope all this was helpful to you somehow. I made a racing game in college… while also trying to figure out how to use the engine for the first time. UE4 is much easier than UE3, though. Good luck!