I'm not sure if Masklings will have boats, depends on how difficult the AI is to write. Sea-born invaders is something that I'd like though, the sea people were a major element of the bronze age collapse.
A Viking race of high-health beings that come and raid your settlement, destroying a few buildings in the process.
Also, if you were to add some kind of upgrade tree for towers and archers, where they could either get better, or be fire archers, would be cool. The fire archers could do massive damage to ships and wagons, as well as structures.
Since the map has edges, I think it would be cool to spawn them from there. You could have raiders as well as trade caravans. For traders, you could require a special trade outpost building only available once you've made contact. They could be like the Harrapan trade settlements in Sumer.
As for the raiders, you could step up the raids as the map becomes cleared of barbarian tribes, or some other metric of game progression. Maybe they could even try to form their own colonies in addition to capturing yours.