It is not often I run into such a visible example of what I would like to see in a 4X game, but I think this documentry on The History Channel shows one:
"Modern Marvels - The Alcan Highway"
So what does this have to do with 4X design? Getting away from the specific engineering of the project, it has a number of interesting logistical and strategic elements that I would love to see come out naturally in a game. Namely:
At the time, Japan was a major force in the Pacific Rim, but only in terms of naval force. There were worries that they would attack the US mainland with the major worry being Alaska. Alaska was easily accessible, poorly defended, and geographically isolated from the mainland. The only way it could be reenforced would be via moving troops in ships, at which point the troop carriers would have to content with the Japanese Navy, which could probably stall them long enough to dig in. So here we have element one: strategic footholds that getting reinforcements to is difficult (most 4X games treat travel as very quick).
So the US&Canada decided to build a highway that ran from the lower US, past several Canadian airfields, to Alaska. This was a major project, not something you can do over and over. So the scale is another issue, 4X games, it only takes so long before you can tile the entire map with roads or rails, so the cost of infrastructure tends to be high early on and dirt cheap later on. I am not sure how to offset that, but something needs to scale the costs so that major strategic builds do not end up tiling the whole map.
Lastly, in order to actually run such a massive project, it was cheaper to build an entire associated oil infrastructure, including wells, refining, and distribution, right off the road construction. This would be another interesting element, logistics where getting resources to massive projects other then 'use 50 shields' is a real consideration.
So ideas ideas ^_^
I think this would also be a good example of how a 4X game could be moved into an MMO type domain. Logistics has the potential to be quite complex, a game unto it'self. In EVE, even though it was painful and boring, people still would play pure logistics roles for years at a time. Imagine what it would be like if someone built a game where logistics was interesting rather then tedious?