Principate
Saint Titanfall
Immigration in that necessary scale wouldn't help that much, there aren't hundred thousand of skilled workers which only wait to immigrate to Japan and low skilled or non skilled would be a burden to the economy.
Japan will just end as important case study how a first world country can deal with a shrinking population. And Japan isn't gonna be the only nation in Asia with that "problem".
You don't need hundreds of thousands of skilled workers automation replaces that need largely anyway. You need carers people that can look after your continually aging population while providing input to your economy while doing so. The economy won't exist in the same manner it is (which wasn't going to happen anyway) but the situation is stable and the worst of the negative feedback loop is avoided. There's a good reason why Japan is the among the worst suffers of this.
This happens already in many western countries around the world. It's why immigrants make up such a large proportion of carers in many western countries.