I wanted to build upon the idea of city layout influencing economy as well as defense that we touched on in another thread, and give out a strawman of what might work.  My goal is to have where things are positioned in your cities matter for more than just defense.  The idea that we collectively got to was the idea of markets, which produced nothing by themselves, but enhanced to productivity of any square around them.

1) All "producer" improvements (gardens, workshops, etc), including specials (farms, wheat, stone, trees, etc) become a size of one tile. 

2) There are "producer" improvements for all the basic resources, and each producer tile needs it's own kind of base terrain to work on (gardens need plains, workshops need trees, stonemasons (make 1 ore) need stony ground, etc).

3) Markets consume one tile, and enhance the production of the four tiles to the left, right, above, and below the market tile (making a cross) by 50%.  In this case, say you have a layout like below, where "G" is garden, and "M" is market

   G

G M G

   G

In this case, each garden now produces 1.5 food, and the total aggregate produces 6 food.  It's a slight improvement over just replacing the central market with another garden, which would be a total of 5 food.  However, you pay a "penalty" in that you cannot arbitrarily place these improvements (which may be a defensive advantage to have another layout, say 5 gardens in a row to block access to a choke point).

4) Markets are built "instantaneously", on the round that they're placed, allowing the building of things around them the same turn, which is a departure from the additive growth mechanic currently in place in the game, where you can only place an improvement next to a currently built tile.

5) In the same vein, plazas are a 2x2 tile improvement, and improve the production of improvements in the tiles next to the plaza (with the exception of the corners) by 100%.  A fully built plaza would have producer improvements on the 8 tiles surrounding it, for a total tile consumption of 12 tiles.  The configuration would produce the equivalent of 16 tiles of production, however, for more of an improvement, but at a cost of more planning required (and more specialization) to gain the benefit.

6) Districts are 3x3 tile improvements, and increase the 12 tiles surrounding them (corners excluded, again) by 150%.  In this case, 21 tiles are consumed by the configuration, but 30 resources could be produced by the configuration.  21 tiles represent a significant investment in that city.  I can't remember the way the progression goes, but it's around 30-40% of the total maximum tile allocation for a L5 city, as I recall.  If you have a district, you get significant bonuses (each producer improvement in an L5 city would be making 1 resource x 5 (for L5 city) x 2.5 (for district bonus) x 12 tiles = 120 production of resources per turn. 

This mechanic gives value to position of economy buildings, gives a tradeoff between economy and defensive utility to tiles, and encourages specialization of cities, because a city cannot ever support more than 1-2 districts when fully grown out (and there are mechanics being discussed to limit the number of L5 cities).

Okay, that's the idea.  I know there are some catches; the big one, as I see it, is that it doesn't REALLY encourage specialization currently, because you can build different producer tiles around the district (say, 6 ore producers and 6 gardens) and you'd still get bonuses to each.  Maybe the bonuses only apply to the most populous tile type around a district, or what have you.  There are probably other gotchas; but I see enough benefit to this idea to be worth discussing.

Winni

[Added 05/03/10] I should also say that I think this works with "special" tiles as well.  Reduce the size of the farm from 4 tiles to 1, and if you build a farm on that tile, it produces, say, 4 food instead of one.  It then gains from the benefits from placement next to a market/plaza/district in the same fashion.  I realize the current philosophy is to make specials a 2x2 tile (including forests) to limit the number of such placements (by consuming more tiles), but I think this may be a better way of going about getting to this mechanic.  I'm also now, after thinking about it for a bit, convinced that the bonus should only apply to those producer tiles that the market tiles are assigned to; ie, when you place a market, a pop up window says "what type of market? food/materials/ore?", and you assign that market.

The other thing I thought was that markets should automatically grow to plazas, and districts, with city size (L1, to L3, to L4).  As the city levels up, the interface would bring you to your market, and say that it needs to be "rezoned" to expand, and you'd be called upon to demolish old producers around it to make way for the expanded market.  You can specify which direction it goes, but it stays in place.  You cannot place a district newly placed; they can only be grown from markets.  If you didn't start with that city market back when it was an L1 city, you won't ever get a district in that city.  This prevents the mechanic where people just wholesale bulldoze a city and start city planning anew, complete with placing districts and such in L5 cities.l