Re: Campaign games,
Most wargames follow this fixed structure. At Start OB, specific reinforcements arriving as they did historically.
Indeed, the same is true of some HASLs; PB especially. I think this just emphasizes how unique the
other HASLs are.
Most of the advanced ACW games have written orders requirements, that might or might not be accepted and followed if they are delivered. So there is spice in these games. They are not all fixed, and pre-determined.
The RSS series Gettysburg,
This Hallowed Ground (or as some like to call it, "This Heavy Box"!) has the usual historically-based times and places for reinforcements to enter, but also allows an option (IIRC) for the times to be randomly modified a bit, so as to add back in some excitement and fog of war.
However, the even more interesting variant is the one where units enter at the historical time and place... but not necessarily the historical unit! When a unit is scheduled to enter, you do a random pick from the remaining available reinforcement groups to see which one you get! This is a great way of re-creating the uncertain strategic background surrounding Gettysburg; it began with a random encounter between two random small elements of the armies, and quickly ballooned as more units piled in from all directions. But there was no master plan behind which scattered forces approached the town from what directions, and on arrival they were just tossed in where they were most needed at the moment.
Now, the downside is that the terrain is still the same, and the features that Meade and Lee considered important are still important for the same (historically accurate) reasons. So even with the "random reinforcement schedule" added, there are distinct limits to just how different a playing of the full battle can be. But at least you're getting shoehorned by historical accuracy and not by a script...
Furthermore, as Brian said, the written orders and the random element to how long they will take to be implemented add a very different feel to the CG, so even if all the forces and terrain are known, the battle can still develop in unexpected ways.
I would not want a battle of Gettysburg game where I was free to buy my own OB from a list of possible units. One of the reasons it works for a HASL is because the units are all generic anyway; that 4-6-7 doesn't represent the 3rd squad of the 2nd Platoon of Battalion 3 of the 295th Infantry Division, or whatever, it just represents "a squad of rifle-armed men". I could also see doing a simillar sort of buy-your-OB campaign game for Panzer Leader, because the units are similarly generic. However, I assume (and I think someone stated) that the counters in TDC are each labeled as a historical unit, so I don't see it working well there.
John