Heya!
Interesting topic, and with the amount of time spent looking at islands, I totally understand having even more variety available.
As some others have mentioned, there are existing modding projects (e.g. by Taludas) to reskin islands for another biome.
From our side, however, there won't be any new island models for Anno 1800. The process for creating an island takes a while and involves multiple departments, it's definitely only higher effort side of things. Plus, going by the comments, one or two islands likely wouldn't quite cut it (and islands would not classify as "cosmetic" content, they are one of the key gameplay elements of Anno).
Far more importantly, however, is, that we are at this point investing our development resources into our next project. That is our top priority.
So, I'll have to shatter any dreams of a map pack DLC, I'm afraid.