Just have Ichikawa help with the world building, adding more types of Tsukijin and help the design team and give them ideas for more weapons, there's already a variety of them in the series, like the sawblade that Antarc uses, the scythe of Ghost
and such, you could go from there and make more crazy shit, you could also implement a quest system, helping the gems in patrols, gather mats to help Red Beryl make more dresses, help Obsidian to craft new weapons, there's a lot you can go from there. The fighting system would be pure hack'n'slash a la Platinum Games, it seems the right way to go just based off from what the series gives us. The story would just be like the one we have right now, but with more story telling and more diverse, telling us more about the gems and developing their character more. But a game seems like a far away fantasy, but if it's really far away, I guess by that point the story would be more fleshed out and more easy to give us more material, maybe even add a moon map to explore and to go around the moon base bismuth
, I don't know, overall it should be great if it's Houseki related.