Pranking people is fun, and nothing is better than whipping out your phone and texting your significant other with a suggestion that would make their jaws hit the floor. Texting your boyfriend 'how's about a baby?' would either send them into an ultimate panic, running for the hills, or maybe they would be pleasantly surprised and keen to start a family with you, despite it being a joke on your end (right?).
There's no way to know for sure what your boyfriend's reaction would be unless you gave it a try and texted him randomly with the ultimate question to test his personality.