I love the campaign mode, it like a mission that you have to do to be come rulers of the world, instead of just random kill everything (sandbox mode)
I also want there to be a special item that you get for completing the campaign which you can use in sandbox mode\multi player mode.
As long as its not too powerful.
Eg. Necklace that adds +1 mana per turn
The only problem with campaigns, you only ever do them once, but you play sandbox\multi player mode MANY MANY times. But if campaign mode gave you items to keep (maybe one item per level you play the campaign on) in the other games, you would be more likely to play it a few times.
But there would have to be some 'anti-cheating'. Most games theses days, you have to play with an internet connection, and theses 'locked' items are linked to the account.