Archipelagos are a chain or collection of nearby islands. These may be formed by volcanic activity along the border of two tectonic plates but could also be formed by continental fragmentation. The islands of an archipelago may differ greatly in size and shape. They may be close together, or relatively far apart. There may be bridge systems that link the islands, or ferry routes that connect them. The islands themselves may be populated or may be deserted. There may be treacherous waters between the islands, leviathans that patrol the seas, or pirates depending on the world.
Designing Fictional Archipelagos
Climate & Terrain
When designing archipelagos, first consider the general climate of the area. This will allow you to imagine in a general sense what the islands themselves may look like (are they tropical, volcanic, rocky, etc). Island elements should be considered, as well as coastal elements and elements relevant to the general region or biome. Designing the oceans or seas between the islands is equally important, and ocean elements may provide additional ideas for things to include. Creating a map of the region may be helpful.
Transportation by ship may be the predominant form of transportation between the islands (air travel may also exist depending on the era or genre of your world). Islands that are closer together may have bridges between them. Consider how easy it is to move from one island to another and how long the trip may take.
People & Culture
Culture design of an archipelago may focus on the ocean and sea travel, as this likely plays a large role in the daily lives of those that live on the islands. Important symbols may derive from sea creatures, common foods may be fish or seaweed, and religions may have the ocean as a central or important aspect. Volcanos may also be featured if the archipelago is composed of volcanic islands.
The archipelago may be the site of an island civilization
Governments & Factions
If the islands are populated, there may be different governments or factions that exist within the archipelago. Consider the territory each lays claim to and how they may defend these waters. There may be warring navies and air forces.
Trade & Economy
Different islands may have access to different resources and inter-island trade may be important within the archipelago (as well as with the outside world). Consider the trade routes between islands, the companies or individuals that participate in trade, and the trade goods exchanged. Food, materials, and precious items that come from the sea are likely common while items that are not nearby may be exceptionally rare and expensive because of the longer, more involved trade routes. Piracy may be a problem.