Does it hold true to derive rulings based on the the Imam's opinion, and what are some examples of that?

Both ancient and contemporary jurists have derived rulings for new issues based on the opinions of the early imams, and this is sound and permissible. An example of this is the disagreement among Shafi'i jurists regarding the validity of the iqamah (call to commence prayer) performed by someone who does not have the authority to do so, which is derived from Imam Shafi'i's view that it is impermissible for one person to deliver the sermon and another to lead the prayer.