The second part that makes this thread great is all the advice given by other users. Below are a few excerpts. Be sure to check the thread out for more great advice.
In my experience, the cooler a table looks (static) the less playable it is. I’ve seen tables you would DIE to play on… once. After a few games, you’ve got every angle figured out and you never go back to it. If you want to compromise, split the table into three sections; two flat and one river. That will give you lots of playable options. You may want to make a third flat section so you don’t HAVE to play with a river every time.
Tips I’ve learned along the way -Paint the roads on it so that the line up no matter how it’s set up. Then make the bridges and buildings modular so you can switch it up. If you’re serious about it, just get the boxed city set. Sand and paint the flat table sections black, then base the buildings on equal sized MDF with the building recessed 1” from the edge of the MDF. When you lay out the buildings is a city format, the black base becomes the roads between the buildings and the 1” around the buildings can be painted up as sidewalks. Gives you a great final look.
WD has some cool urban tables. There was one that had a sunken highway instead of a river. Very cool.
It looks cool, but you'll get bored of it, unless you plan for enough pieces that can be moved around... and I think you could easily, with rubble piles or buildings or little bridges over the canals or whatever. - Lil Tweaker
Also, I agree with whoever said it -- this is going to be a huge project, probably bigger than you realize. Even creating a relatively simple 8x4 grassy board with a couple hills and rocks and things is an enormous task (I know because I did it). Fun, though, so knock yourself out if you're up to it... the best piece of advice I can give is to not rush, and be as precise as you can be with everything, and think everything through five times before you do it while buildings. I made a lot of mistakes with my board just because I didn't plan how things were going to go together well enough... for instance, plexiglass resin melts through just about everything except for aluminum foil. - The Salmon Thief