Short answer
They should discuss important expectations involving faith, finances, family, work, children, residence, health and communication.
Islamic Guidance
What Islam teaches
Islam encourages honesty and good treatment. Important matters should not be deliberately concealed.
Practical Guidance
What you can consider
Create a practical list of important topics and discuss them calmly before finalising the marriage.
Sources
Qur'an 5:1; Qur'an 4:19; Qur'an 30:21.