Hi Martin,
The Fiori apps are designed to be used to perform a single function (1 function and no more than 3 pages. I would suggest that perhaps migrating to Fiori may not be appropriate if you expectations are much higher.
Fiori apps are not designed to replace more complex apps one for one. Their audience is users who only need simple functions in simple apps. The most capable users who need complex features are going to need something else.
Having said that, if the only deficiency is something like the team calendar being visible, you might be able to extend the app (customize it). Not being familiar with what coding would be required to add team calendar functionality, I do not know what effort it would take. It may also end up causing visibility issues when on smaller mobile devices.
I would suggest reading more about both the individual apps and Fiori as a whole, before making a decision on whether they are useful in your circumstances. It does not sound like the customer will completely eliminate the need for the present WD application, but perhaps 80-90% of the current user base would like to have a Fiori app to create their leave requests. That might be a better plan for them.
Regards, Mike
SAP Technology RIG