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.

For something that I think an app or service could actually solve, I wish there was an online database of PC hardware where people could submit hardware compatibility reports for various OS’s as well as workarounds or solutions to get things working.

Something that would be a cross between PCpartpicker and protondb in terms of functionality. For example, if you’re buying a new PC you could go to this hypothetical site, list out the hardware your considering (or if it’s a prebuilt pick that), and get a list of user reports for compatibility with various Linux distros, BSD distros, and Windows versions, alongside suggestions for how to get things working. Or conversely, pick an OS and filter your hardware options by that.