It was my mistake to use the word storyline, but I tried to explain it by placing the word psuedo in front of it. Obviously MTW2 can't handle character driven 12 part story arcs, what I simply mean is the implementation of missions that one tier above being completley random.

For example, in a Dwarf Campaign:

The Dwarves start the game from turn 1 with a mission to take Karak-Zarn (or whetever the appropriate city is), which was invaded by orcs/skaven hundreads of years ago. You could ensure this mission is given straight away by giving it an immense priority stat, and having no minimun turn to start. The duration would be 50-100 year, and the payment would be lots of cash and the trait Defender of Karak-Zarn, as well as a nice in-game text message about how the dwarves celebrate.

The player doesnt have to do the mission, or they may attack Karak-Zarn for their own reasons, but the point is that the mission is a deliberate part of the Warhammer world fabric, as opposed to being completley random. There arnt that many possibilities available with the game engine, maybe lots of assasinate missions for Skaven and Dark Elf, lots of attacking missions for orcs, and maybe more diplomatic missions for the Empire (forge an alliance with Bretonnia, Dwarves and High Elves?). I forget if there are missions that involve merchant trade items, but that could be used by Skaven to take a particular warpstone site, Undead to take back a pyrimad etc.

An interesting question is whether the completion of one type of mission can be set as the starting condition for another mission. If so, then it would be possible to have a string of missions that related to each other, and that you attempted one at a time, in a sort of a pseudo-storyline. This part I dont know about and doubt, but would be great, otherwise you could give these individual story missions staggered minimun start dates so that they happened over time.