More Slugcats Expansion is an upcoming expansion for Rain World 1.5 and will be expanding upon the base game with new Slugcats and campaigns, lore, regions, rooms to existing regions, as well as new mechanics and game modes. The official MSC website has the full description of the modded expansion as well as some exclusive teaser content, which can be found here: http://eggzero.serv.pink/rainworld/web/web.html
The expansion features 10 regions as well as changes to existing regions, resulting in roughly tripling the screen and room counts. It features an ost with over 40 tracks. There are two new gamemodes, Safari and Challenge modes. Safari mode allows you to take control and possess any creature in the game, whether Vanilla or added with this Expansion.
The official More Slugcats discord features discussion and teasers on the Expansion: https://discord.gg/5NFZ4px