Yes, many attempts of a MP campaign have fizzled out, but dont get disheartened. I do know of some that did take off and went through to the end.
Some things to remember, though.
Dion is right, these things can become tremendeously immersive and end up taking a lot of time if you are not careful. So the numer one rule is to really KEEP IT SIMPLE.
And that means in terms of rules, how you are going to calculate the outcome of moves, which unit types you allow in, and how many players you allow in. If you don't just have "dedicated friends", but want it to be open to people from the community, make sure that if one player dropps it does not stop the progress of the game. And you'll be amazed how long it can take to complete a turn. I would reckon that one turn per week is very fast going if you have 3 or 4 active players. It may also help to have a suitable victory condition, i.e. rather than fighting to the last province, it could be "most powerful faction after 6 month" or so.
That said, it can be a thrilling and exciting experience to run a MP campaign.
Just remember with whatever system you come up with. If it you think it is a massive simplification compared to the SP game, it is probably just manageble when it gets into full swing.
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