Expense tracker, bill splitter
Like SplitWise - for a group trip or project: let group members log expenses, upload receipt photos, automatically calculate who owes what to who.
For a household group, this could work ongoing for food, bills etc, even better if expenses can be categorised.
This is easy functionality used in loads of apps, and really useful - over 5m people use SplitWise alone. Why not bring it all into teams