Using the assigned To Do's to aid tracking of individual task status. Some To Do's require action in sequential order, are dependent upon completion of another individuals tasks. Having the "part of", dependent on, or even subset of linkage options would be helpful to link To Dos together & highlight dependencies.
Thank you for the request. Based on this use case, we would suggest using Features (instead of To-dos) based on the greater power and flexibility that you would have with features. This would allow you to link features together, and you can still make them part of Release phases.
At this time, we do not have plans to add dependency options on To-dos based on current priorities and historical feedback. We hope you can understand.