Wow great idea! but it may be kind of annoying for some players if they click "play online" and no one is there to join their games. Sure - if it happened it would be sweeeet - and I'm sure that people online looking for games are going to do anything possible to play a game - even if they are a meagre army against a formidable force - they would most likely do the thing that a small force would do - even withdrawing off the battlefield with the majority of the troops and screen the enemy with some, thinking of the big picture they can't see (unless that feature was incorporated). The opponent online can have a brief overview of the big campaign map to get a feel of what kind of battle this is and then go into it. This idea obviously makes for some frusterations where the opponent could just do annoying things to piss you off with no semblance of a strategy - in which case you as the host have an option to "Boot player/Switch to AI Opponent".

:D

I also agree that without MP the game dies as soon as the peopel finish the MP or the game's features and graphics get outdated.

Why do you think Starcraft, Halflife and such games are so impressively STILL played these days? It's certainly not their graphics. It's their online ability to make great MP battles with nearly flawless gameplay balance. Without online battles FPS would never have been as abundant as they are today.