cityplannerplays dropped a prophecies of ash video :)
from what i’ve gathered
the new islands are at the edge of your latium map, like top right or top left corner
as you sail to the new area you will see the volcanos, and the settlement cost is a LOT since the islands are HUGE
the new islands have like 20 river slots and mountain slots and a crazy amount of fertilities, so they can become powerhouses for the rest of your islands
the island still looks natural despite being so big, and you are introduced to new story characters, new deity named Volcanus i believe
all islands now have obsidian (from the volcanic eruptions) that you can turn off, but each eruption replenishes the supply if it’s low
i think this was the best way to introduce a DLC where you don’t have to worry about switching regions for trade or anything of the sort, and your current islands help you establish the new ones. get your $ up so you can afford it, at least! but overall it seems the other DLC will switch you to new regions like Albion where you can still have trade routes established but settling will take longer.