I remember this being discussed in the past, and I agree that it could be convenient.
However, fixed time cb, e.g. 3h classic or 1h blitz, is about using the given amount of time to create a big city. Some of this time you need to spend on planning, some of the time you spend on doing actual construction and some of this time you spend on taking breaks if you need it. If its possible to pause you get a situation where its possible to buy yourself time for decision-making along the way, which I don't think is something you want.
A solution could be to have a fixed break midway in the 3h games. Same idea as in any sports game. Could be possible without messing too much with the game mode, and I would think it is easy to implement.