Two that come to mind, for me:

A todo-list/task list/scheduler app, with the following requirements:

• Tasks are input with a due date and priority
• Tasks can be single-occurrence, or recurring
• Recurrence can be static (weekly, monthly, yearly, every X days, etc.) or dynamic (X days since last completion). Dynamic recurrence in particular is the thing I have had a LOT of trouble finding.
• Keep logs of completions
• Multi-user (any user can add or complete tasks)
• Self-hostable

A finance/accounting app

• Ledger-based accounting, I.E. a distribution moves money from one ledger to another, and you roll up distributions by ledger to get balances for each ledger, and transactions consist of one or more distributions
• Ability to create reports, by combining different ledger balances and/or search criteria (E.G. date ranges, or groups of ledgers)
• Ability to mark transactions as reconciled or not
• Ability to attach supporting documents (E.G. receipts or bank statements) to transactions
• Ability to import transactions from CSV or other exports, from third party systems.
• Ability to setup reusable templates for importing, regarding which columns to pull which fields from
• Ability to use regexes or similar if parsing is required to pull values out of, within import templates
• Ability to use regexes or similar to choose the ledgers for a distribution, within import templates
• Ability to manually fill in whatever fields couldn't be automatically determined, during an import
• Ability to warn about possible duplicate transactions/distributions during an import
• Ability to group distributions together underntge same transaction, during an import
• Ability to exclude distributions, during import.
• Ability to edit any data element in the app, and track version history.
• Self-hostable or standalone storage

These are both concepts for which a LOT of apps exist out there, but for which I've never found one close enough to be worth using, despite searching off and on for years.