We are still tinkering with reflective materials. However, actual reflections (except glossiness/specularity) have a major impact on performance, especially since these reflections need to be computed in realtime in Rising World. We have added screenspace reflections, which aren't that costly (in terms of performance), but they have some visual limitations. In the long run, that could be replaced by raytracing.
Actual mirrors, on the other hand, are different. They also require realtime reflections (and we can't work with screenspace reflections here), but since they only consist of a single surface (and it's unlikely that people place lots of them in their house), it's definitely something we take into consideration
In short, mirrors (as objects) may be added in the long run, but when it comes to reflective materials, we will rather stick to some basic reflections here.
Yes, that's planned It's our intention to add snow to higher elevations, and that would also affect the player temperature.
If you guys work on mirrors, please, PLEASE, allow implementing portals they're literally based on the same techniques.