Grommit won't be the first silent third-person adventure game protagonist. The Neverhood's Klaymen never spoke in-game, and they pulled him off very well.
I'm hoping to see the distinct difference between Wallace and Grommit's inventing-related philosophies. Wallace creates decidedly complex inventions and setups designed for specific purposes, while Grommit is more practical and has a habit of finding alternate uses for Wallace's existing inventions. Wallace tends to fail to see uses for his inventions beyond the originally intended ones. This would make for great puzzles.