Yes, of course, you can create such processes in Process Builder.
Let's have a look at the xCP Grants Management Sample Application. I think, it is contains a few ad hoc processes.
For example, Start Review Process: a user (Grants Manager) decides when start this process (if all the necessary information is collected) and who (which is of Grants Reviewer) is involved in Review process.
Another example is Request Documents process. On the one hand, this process can be started many times for one Grant Request. For other Grant Request, this process can be started just one time or not stared at all. It all depens on the Grants Manager.
Thus the main process can have several scenarios execution. But no one can predict in advance the direction in which the process will flow.