I read an interesting article about how Eve Online handles the server load caused by an unusual high number of players in the same area.
Their main trick is to slow down the gameplay when the server is overloaded (likely by reducing the number of ticks per second on the server). I could easily do the same, I just have to modify a few places where I assume that the tick rate is globally identical and I would have to inform the client that everything now moves slower. But I wonder if this would really reduce the load much in my case, because I assume that a player which has more time to do things will also try to do more things, and they will still send move- and action commands with the same real-time frequency.
Another thing they mentioned is the ability to temporarily move areas from a weak cluster machine to a dedicated server with much stronger hardware, but unfortunately this has a critical implementation flaw: It disconnects everybody on the server. It also doesn't happen automatically and needs to be triggered by an admin.
Currently my game is not cluster-ready, but in the long term I plan to make it possible to put each map instance on a different server, preferably fully-automatic to keep the administrative overhead low (I keep this in mind, but I won't do anything in this regard until I have a player base with a size which actually justifies the effort). When I do that, I should look into making it possible to also transfer the running instance together with its players to a different server, preferably without them noticing and fully-automatic.