Which would reduce visibility, mix that with ice and that could provide some challenge at Sonic's (and Shadow's if he's playable) speeds. Then the further you progress the snow starts falling more and more until it becomes a blizzard. Like with the blizzard idea above, the start of the level could be clear.
Now the weather wouldn't have to change constantly, like at a certain point these events would be triggered (like Escape from the City with the truck for instance). Now I would say lava (or magma) and volcanoes, but I think those have been used enough (mostly Crisis City, Chaotic Inferno Zone, Flame Core, etc.). Or even escaping sand-storms, blizzards, avalanches and cave-ins (even to avoid falling rocks with quick-step). Like instances where quick-stepping is required to dodge lightning strikes or Sonic (or other playable characters if there is any) needs to escape a flash-flood in a canyon. I'm thinking the addition of weather and environments changing throughout the levels, depending on the location of course. Now if the next game was to be closer to Unleashed (in terms of daytime level length and real-world inspired locations) than Forces. And maybe the additional playable options of Shadow, Silver and Blaze. And it was going to be more like Unleashed, with longer and more real-world Modern Sonic stages. But what if Sega announced a new Sonic game, with footage. I know there will probably not be a Sonic Unleashed sequel.
