The thing that makes S&D hard is also why some of the other things you mention would be hard, so it's the common underpinning that needs tackling sometime anyway. Specifically, the underlying issue is that the AI has absolutely no concept of central control, or really of cooperation. In the original BTS AI, every stack makes totally independent decisions about what to do, so essentially it's locally greedy but has no concept of a bigger picture.
For S&D this manifests in a few ways:
1) If there are multiple stacks in an area, which should perform attacks and which should take up surrounding positions to aid another stack's attack?
2) When should an attack happen - better to do it immediately, or to wait for some other stacks to get to surrounding positions (even on the same turn that is)?
3) Since the AI is very SoD-oriented, when to break up a large stack to allow independent movement (and then when to put it back together also)?
However, for things like a pincer attack from a different direction, or sensibly coordinated pillaging to support an attack, much the same considerations would apply - basically anything that requires more than one stack to coordinate activity falls into this category, and currently the AI has no good mechanism to allow for this.
Since about V26 I've been making changes to alleviate this situation, mostly with the unit contracting mechanism, through which in C2C now:
- workers, settlers, great generals now advertise actively for escorts when needed, and units without high priority work advertise themselves as available (the contracting system then optimizes the hook ups based on how well units meet the requested criteria and how easy it is for them to get there)
- great commanders actively contract for hunters to stack with, in order to go get early experience
- cities contract for defenders, rather than always building them for themselves (cities without high priority builds can also respond to contracts by building units, and in that case the contract broker picks the best placed city, based on the unit it can make, the promotions it would provide, and the distance it is from where it is needed)
- City attack stacks (aka SoDs) actively advertise for attack units (always), healers (if they have none), and a great general (if they don't have one) to join them and keep doing his until they are ready to roll and have left home turf
All of the above (apart from city defenders which the cities always used to just build) used to be handled exclusively by each stack looking around its local area for something to do/someone to join (so SoDs would only build up as quickly as attack/city attack units happened to move close enough to see them and decide to join them, and so on). This has been a big step forward (and the same system almost certainly has further uses), but we need to go a step further to achieve S&D (and other cooperative action) AI. I have something in mind, but right now it's only an idea, and it's going to be quite complex to implement and take a long time of seeing how it behaves and tweaking in response to that to really make it work well.
