Yep... the limitation of the "matchup" game design would be that you have to have to have a reasonable campaign between the order and destruction armies... I don't see skaven and wood elves being quite the complelling face-off that say, Brettonian and Skaven might be.

Of course, the other way they could do it would be to create a new face-off with an existing race.

For example, a new campaign between Empire humans and Skaven, with a different city serving as Empire 'captol' - say... Middenheim. ;)

Or a campaign between WoodElves and Greenskins.

Actually I like that idea... That way they only have to balance one new race at a time.