Ever since the concept of EU citizenship was formally introduced in the Treaty of Maastricht, a lively debate followed as to how European citizenship, in particular in relation to the ‘traditional’ concept of national citizenship, should be viewed. It has, for instance, been described as a form of composite citizenship, in a sense that citizenship is composed of various statuses, rights and duties depending on the legal order to which the citizen is subject (Van Eijken 2015). The idea of composite citizenship is still very much linked to the definition of EU citizenship in the Treaty, which stipulates that EU citizens are nationals of the Member States, which has an additional value to national citizenship. The question is how the formal, legal conception of EU citizenship as can be found in the Treaty relates to a number of developments in the EU, like the upcoming Brexit, the Rule of Law crisis in a number of Member States and the case law of the European Court of Justice, attaching a fundamental status to EU citizenship. Considering these developments the question arises, whether EU citizenship should not be perceived as, or develop into, a more open, fluid concept, which has gained an independent and not merely additional status to national citizenship? In this paper the (legal) possibilities (clues) and constraints will be explored, which can already be found in old case law of the European Court of Justice and in the literature, with a view to develop a more open and inclusive concept of citizenship.