Is Nirvana the goal for all Buddhist? If I understand it correctly, Nirvana is the liberation from the endless cycle of rebirth, thereby terminating all sufferings.

Is Nirvana a goal which every Buddhist must achieve?

Is there any other things more important than attaining Nirvana?
No, Nirvana is not the goal of all Buddhists. According to Sakya and Nyingma schools of Tibetan Buddhism, Buddhist liberation methodologies can be divided in three major classes, corresponding to sentient being's mental capacity:


students of lower capacity seek pleasant living conditions and personal happiness. For these beings the teaching focuses on fundamental principles such as karma, and posits the goal of better rebirth (and better luck in this life). When a being is born to conditions more favorable for deeper study and practice, or in the same life its interests raise beyond basic day-to-day existence, it is now considered as one of the
students of middling capacity, who seek spiritual realization. For them the teaching focuses on personal transformation through removal of mental and emotional obscurations and posits the goal of Nirvana. Once the student removed enough obscurations to have a glimpse of how things really are (tathata), he or she realizes that both Samsara as well as Nirvana are two sides of the same equation, and a projection of one's Thirst due to one's mind having been intoxicated with The Three Poisons. At this point they find themselves on the level of:
students of higher capacity, who practice Natural Great Perfection. For them the teaching focuses on manifesting unhindered compassionate action, union of wisdom and skillful means, and does not posit any goals.


The three capacities model should not be confused with the three yanas model (Hinayana/Mahayana/Vajrayana) which can be seen as three levels spanning the second and third capacities, nor with the three schools model (Theravada/Mahayana/Tibetan Buddhism) which are three alternative paths, with beings of all three capacities found in each school.

Another model defines goals according to one of the Six Worlds the being finds itself in:


For beings in Hell realm the goal is defined as Escape from Suffering
For beings in Animal realm the goal is defined as Wisdom
For beings in Preta realm the goal is defined as The Only Possession of Real Value
For beings in Human realm the goal is defined as Liberation
For beings in Asura realm the goal is defined as Perfection of Excellence
For beings in Heaven realm the goal is defined as Protection from Impermanence


There is also a nine-yanas model that provides a finer classification of enlightenment contexts, with a slightly different provisional goal utilized in each. Please note that all models exist solely for their explanatory value and are vastly simplifying reality, which is a lot less pigeonholed.

To answer your second question, is there any other thing more important than attaining Nirvana, this is like a school student asking, is there any other thing more important than finishing school. For the school student to finish the school is the most important thing. Nothing else can happen until then.