Is expiation obligatory for saying "Insha'Allah" (God willing) without fulfilling what was said, knowing that it is uttered spontaneously and without the intention of making a vow or an oath?

The statement, "I will do such and such, if Allah wills," does not entail any obligation. Rather, it falls under the category of remembrance (dhikr) or seeking blessings by mentioning the name of Allah the Almighty.