There is a Castaways 2 in the works, but it is quite a bit different from the original Castaways.
The expansion to Castaways 1 was already added into the game, so you should see missions 6 and 7 (which are the expansion). Originally Castaways only had 5 missions.
I'm glad you're enjoying the game and I wish you luck on completing the rest of the missions Mirage.
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