BulldogLowell, Well mentioned. It does not come to my mind. I'll consider it now. But I have one more question in my mind.
All the things such as call to connect to MQTT server will get initiated with instance of library. And I'll have to call that setClientId(const char* clientId = nullptr) function in void setup() function of my sketch. It will be too late then, however it will work. But in the mean time, I'll get two connection notification for single unit on my MQTT server with two different client ids.
For more information, have a look on to my library at github.