It would be extremely useful to be able to add a custom field that can only link to a single task.
This would enable intuitive linking between tasks(records) where there is only a single logical corresponding task(record).
One example is where a certain task is designed to be a single step in a process where that task can only be performed for a single project at a time. Naturally, the "Project" field of the task should be limited to linking only to a single task(record) that represents the entire project within a separate Projects list.
Consider the potential this brings to automation where the referenced task's values could easily be used in automation routines. For example, the automation for a project-related task could assign a newly created task to a project manager(user) that isn't referenced in the immediate task or list and may change depending on the project - that project manager(user) would be stored in a custom field of the task(record) that represents the entire project.
The same concept would apply to numerous use cases that have historically been handled very efficiently using relational databases that enforce one-to-many relationship constraints.