So for the most part there are no real relations between the continents. I thought of a way there could be without the need to create any new rules or features. The only question is "is it allowed?".
So here's how it could go:
Essentially the only commodity you can move across continents is gold in the possession of a noble. There are a couple ways this could constitute commerce. None are easy for players to enforce but they might just be worth trying at some point to some one.
1. Nobles might be intentionally dispatched to serve a certain amount of time on another continent, then return with their earnings. Various arrangements could be made to make this worth the noble's while.
2. Nobles could be sent with a gold supply for a loan which the realm will expect paid back with interest.
Not practical at all, but stranger things have been attempted. Therefore my question, is it allowed?
It's not against any rule I know of, and I can't see any good reason to forbid it off the top of my head.
Doesn't the cost of immigration depend on the gold held by the character?
It does. Leaving one island with 1,000 gold will only leave you with a few hundred coins on your arrival in the new island.
Or less. I have done very little island hopping with my characters, but I was greatly surprised with how much gold was lost on emigration. You may come back with a richer family, but you assuredly won't be coming back with much more gold in your pocket.
Given how it couldn't possibly work, I can't see why it would be made illegal.
As others have said:
Most gold is lost on emigration
Guild affiliations are nulled on emigration
Local taxes allow lords to artificially inflate their credit significantly
It would need to be a pyramid scheme in order for gold to accumulate, but the upkeep and costs of the infrastructure is certain to be greater than the desired accumulation of funds
Having done a lot of emigrating, I can say it would be cost-prohibitive. Which is unfortunate, because this is a really great idea.
I wish there was at least some form of intercontinental exchange in BM. And you'd think there would be, considering that you have some Noble families spread over multiple continents.