How about the possibility of EU citizens to move to the UK and UK nationals to the EU for longer terms (longer than 90 days)?
After the end of the transition period, mobility to and from the UK will be different and will happen under different rules than before when EU free movement rules applied. What exactly those rules will be depends on the outcome of the negotiations.
Nevertheless, even in the absence of specific provisions, the default rules of the EU and the UK would cover the entry and stay for different purposes. On the EU side, national rules and the EU’s legal migration acquis for long stays sets out the conditions of entry and stay in the EU for categories such as students, researchers, some types of workers, family members, long-term residents, au pairs, volunteers etc.
These rules apply in the same way to citizens of all third countries, including the UK when it becomes a third country. For stays of less than 90 days in a 180-day period, the Schengen acquis applies. On the UK side, its national migration regime would apply.