I've suggested this here before. I have given it's some thought and I think there are things to consider. When you are a placing a station in the game, you see the green "ring"(with lack of a better word) around the station which tells you potential passengers. The center of this "ring" is the platform and not the entrance/exit of the station itself.
I do believe with walkways this "ring" should be altered to something more complex since I've seen ridiculous long walkways IRL before. They are not very popular, but I think people do complain less about them than just walking om the street when changing lines. I do not know how you could work this out the best.
This is something I consider for the metro upgrade:
The whole station as of today should be in pieces.
-Entrance/exit.
-Stairs.
-Escalator.
-Elevator(to increase attractiveness for disabled people).
-Big entrance. Something like
this that offers the whole package.
-Lobby.
-Stairs and escalator.
-Platform.
-Walkways.
-Small walkway and wide walkway to handle huge amount of passengers.
-Moving walkway to increase attractiveness
-Different levels all the way from bottom to lobby level.
-Slopes(with moving walkway), stairs and escalators.
Tunnels
More tunnels too choose from. 1, 2, 3, 4 track with good merging capability and better priority for handling of trains.
Kind of went overboard with this post.

Not all of these are important, but I do believe they will ad more playability to the game.