It would be nice, if you're able to create the task (with pending/approval statuses) based on request form, which could be integrated with Profile Settings -> Calendar.
The idea is:
1. User submits the request form with his/her leave notice (sick leave, extra work day, day off, etc.), which creates a task assigned to Administrator with Pending status.
2. Administrator approves this task by changing task's status to Approved
3. When task approved - start and end date automatically populates to User profile -> Calendar and highlights according to it's type (types are written in custom field or request form dropdown).
4. It would be nice to have a pop-up menu with confirmation question - when you change anything in your calendar, or any kind of backlog which would allow to track any changes made.
I assume that it should be the ability of the Request Form - just to make it's settings more flexible and with more options to choose from. We are using the request forms to create new projects from our multiple templates as well as for expenditures report and it's pretty impressive and comfortable to use, though sometimes there is a lack of conditional logic.