the passing could have a 'classic mode' or 'custom mode' gametype where you can play a campaign mixing up l4d1 and l4d2 survivors.
so you could be playing through any campaign from either l4d game with a combination of bill, ellis, francis and coach or rochelle, zoey, nick and louis (for example). obviously the storyline would mess up, but if it was a stand alone game mode it would be fun. imagine the dialogue and banter?! fun times.
 
also in versus mode you could have custom options to dictate the SI allowed to spawn. so you could play the parish with only the original SI from l4d1 or no mercy with all SI from l4d2 allowed to spawn. you get me anyway lol... just a thought

I think it would be cool, but I wonder if all the assets would make it a huge install.  Like, you'd have them complaining about going all the way to the other city only to run into the same problems.  Would have to record a lot of new dialogue.  
 
Would maybe be used to get people to have both games, though, which wouldn't necessarily be a bad thing.