While exceptions abound, I’ve always felt that a divided freeway needs to achieve substantial length before it should be given a 400-series number, and that the beginning of that divided freeway shouldn’t be too far from the beginning of its predecessor.
For example, in the case of Highway 6, IMO, it will at least need to have divided 4 lanes and meet a realigned Highway 3 (with divided 4 lanes at the junction at the very least) around Jarvis in order to receive a 400-series designation.
It’ll be a bit like the 69/17 situation.
Another example: If divided 4 lanes of Highway 7 eventually gets to London* to join up the 401 or 402, the whole freeway from there to Guelph may receive a 400-designation.
* We may ignore Old Highway 7 west of Highway 4.
I know, both examples are very pie-in-the-sky, or at the very least, highly ambitious.