The general consensus on Reddit is

2 paths: either the new romanceable Sims, the Joja/community center or, our kids will be able to grow up

3 becomes 4: 3 current kids, 3 current main locations (valley, desert, island), 3 major galaxy weapons

Mermaids Fruit: Children will be able to leave the house, something with the Pearls, some kind of new fruit or fish