Dyer Island will become a map eventually
Felix, Blight and Elodie are all playable characters directly tied by the game's lore to Dyer Island. It's probably the most lore based location in all of Dead By Daylight.
Dyer Island was a former prison camp, contains Talbot Grimes secret laboratory, is owned by a secret society (The Imperriati, which Felix and Elodie's parents were part of) and was undergoing a renovation with new buildings and restoration headed by Felix himself before he disappeared.
It seems to me that the end goal of these chapters without maps is to release Dyer Island because many of the characters being released are connected to it. Saving Dyer Island for a later Chapter gives the Devs more room to work on updating old maps and its eventual release ensures that the newer characters won't be left without a map because Dyer Island is connected to all of them.