While I am a serious grognard and love 'crunchy' campaign rules systems, I have yet to find a gaming group that would go for this.
The best I have come up with in developing a campaign system that people will play is this:
1. At the start of the campaign, each player gets X number of generic territories.
2. People play games at their own pace and at their own schedule. When two people play a campaign game, the winner takes a territory from the loser.
3. The campaign is over and someone wins when one player acquires Y number of territories.
That's it. No maps, no casualty carry-over, no experience tracking. No "I'd love to play Jim but our armies are nowhere near each other."
Now, that being said, there are ways you can extend this, make it more interesting, depending on the tastes of your players. Potential optional rules include:
(A) Handicapping. If 2 players have a game where one has more territories than the other, the player with fewer territories gets extra points / units / bonuses whatever. This encourages players who are not doing well to stay in the campaign and not drop out.
(B) Terrain rules: If your players like building, you can have them build things in their territories (mines, temples, ports, etc) that have in-game effects.
(c) Commander experience: you can have army generals gain experience, getting special abilities over time if they survive long enough.
Things like that.
prepares to do some regimental-level ACW gaming.