There are a lot of negotiation/tactical games (Game of Thrones board game comes to mind), but they all can be equally long and none have the simplicity of Diplomacy.

IMHO, the fun of Diplomacy is sneaking in little side conversations and hiding in a corner to tell a secret. In person, it's very fun. A better approximation online would be to play asynchronously (like a turn a day) via a website.