Mother as a playable unit. Even if she's a fixed level, her stats and cheap Vengeful Stance would make her useful through chapter 4 at least.

Otherside: Escape. You play as a single created character in maps with Suffering minions, NPC survivors also trying to escape, and sometimes Daughters that fight against Suffering. The goal is to escape back to reality. You'd get to see what happens to people who aren't taken in by suffering, and watch the Suffering/Child-Memory/Mother fight from a different perspective.

Corrupted daughters in later stages spawn in relation to how many you've lost. If you've never lost a daughter, then the only corrupted daughters you see are Hopebreakers, and very rarely at that. If you lost a ton of Soulslingers, then you'd see a lot of Painslingers in the later chapters.

Lore speculation: Violence is a very powerful Other. Mother appeals to ultimate violence to get the Scythedancer, which has the ridiculously powerful The Harvesting.

These are just some thoughts I've had in mind for a while.