What they really need to work on is not the speed, but making sure that Sonic HANDLES PROPERLY at that kind of speed. That was the major problem with the Sonic game that came out on the 360 and PS3. While it was fast, it just seemed like a gigantic mess and Sonic's speed was incredibly difficult, if not impossible to control. In the original games, he went fast, but you could still control him and pull off fun combos. That's what the developers need to understand .. speed isn't desireable unless you can control what you do IN that speed.
I really hope it isn't as uncontrollable as it looks from that trailer .. it looks like they just redid the level from the 360/PS3 game, but kept all the gameplay issues that made that game such a disappointment. I do agree with other posters that they should keep Sonic OUT of the Real world and back into the world that he was originally in, with the themes of environmentalism against the industrialism/destruction of Robotnik/Eggman. Sonic can work in an open world, you just need to keep it .. grounded, in a way, like in Sonic and Knuckles. Hell, bring back Knuckles as a villian and give Sonic someone to fight against.
Seriously, I hope this game brings it back in the right way, but also moves forward to the next gen. But from the looks of that trailer ....