Me three
The most interesting areas for me are ones which connect intelligently with the areas around them. So sewer opening in the ceilings and perhaps a full sized opening to outside would make a good set for me. RWS did this though I find that set small.
I like sewers too. They are the urban dungeon and if, like me, you like a little low fantasy in your module creation they are a perfect hideout for the bad guys.
What I think RWS got right is that a sewer is not a place for people to walk round. Only the very largest channels would have walk ways and then these are for maintenance, not for large groups of critters to stand and watch the poo go by. Nine times out of ten the party should be up to their knees in water and detritus. As well the passages should get smaller as you move away from the main areas until they become unavailable for all but the huge rats. I have not done a city that could have an operating sewer but I have thought about this. I would use placed sounds to indicate where the player was. Market sounds or bell sounds that filter through the grate with shafts of light.
There is also some nice role play for the party when the channels get smaller. There can be areas that cannot be accessed without shrinking first? .... I digress.
I know you did not ask for opinion here but a perfect sewer set for me would have:
A good height range
Less use of walkways and thought about how you get from the walkway to the channels
Lots of channel only tiles
A half or 1/3 width corridor set that were channel only
Lots of places where the outside can be inferred via light or ladders.
For placeables:
Large soil piles
Gratings that cover channel so that grate is the walk way
Wooden fabrications that form large walkable areas over the channel. these would form Islands on which bad guys could congregate.
Sticks and detritus
Other
A walking sound for wading, so the footstep sound can be changed when the PC enters the channel.
Sorry, this is your project I know. I just get carried away when the thought of a new tile set coming especially one as important as the sewer set. I hope there are some useful seeds of ideas in the lists and I am really looking forward to seeing some new screens.
PJ