Trigger Action: Start Date on a task

The more I use the rules feature, I more I want more of it to do.

Currently, I can set a trigger that when a custom field is changed to x, allocate due date to + 7 days for example.

In Development projects, timeline view is mainly used. I would like the action to include start date as well so I can integrate dependencies to other work within the project.

The rule I am looking for:
When custom field is changed to X
Allocate the task start date same as change of customer field and give deadline + y number of days


It would be cool. You (or me) could custom code a rule if needed, that possible now.