I need to develop a travel desk app for my organization. I have created some objects called vendors(who will provide the vehicles and drivers), pricing slabs(where each vendor's pricing slab for vehicles and drivers will bemaintained),travel routes ( in this our organization's travel routes will be present) and drivers. these are the master objects. 
I need the business logics to be implemented like what type of validation rules can be crated,use cases for process builders,workflow rules,approval processes etc.
for eg, whenever a vendor has a contract with our org an email should be sent to the travel admin of our company using workflow:email alerts.
similarly i need more use cases in order to develop the app.

Please share your thoughts. Thank you in advance!!