Here's my idea's intro:
You woke up one day in the plain meadow of a strange world when your last memory was actually you falling asleep in your own room... You thought you're still in a dream when suddenly an entity made entirely of pure light speak in all her divinely voice at you: "This is not a dream Noah, the world in your past life has been blown to bits by a random stray asteroid one night, and i decided to isekai'd one soul to save, which is you, to come here and be my chosen one."
The god explained that she was bored with her current creation and wanted to press reset, but take some inspiration from the god in your world ~especially on the Noah's Ark story. So she wanted to create a homage of that favorite scenario here in her own world. Unfortunately she was unable to find a promising breeder-prophet to convert in her own world and be her Noah, so she decided the lazy way out is to snatch one soul when the god in your world was also bored and press his reset button (but the god in your world is more of the psycho egomaniac type, while she admitted that she's more of the lewd fertility goddess type).
Now she explain to you about your grand task as "her Noah" (name later can be changed) is to explore her existing crumbling fantasy world, study the flora & faunas that she has created, their strength & weaknesses, their traits, their potentials, their ecosystem and compatibility / synergy with other species, make contact with the current humanoid inhabitants too and maybe try to convert some that you like, ultimately you should gather resources to build your ship and fill it with all the species & characters that you judge worthy to repopulate your new postdiluvian world. All this while she prepares her flood world-reset formula.
Game will be divided into 3 phase / timeline: Before Flood > During Flood > After Flood
which is basically early game, midgame, and endgame.. all with their own mechanics,
but if i have to simplify each phase as a core concept it would be... Build > Voyage > Repopulate.
If you're familiar with Adam & Gaia and Portals of Phereon, you will see that they have a huge influence in this game idea, but with biblical Noah's Ark twist!
Here's attached a rough table of how i mapped the progression & mechanics for the game concept: