Nation-states are a specific type of state, in a specific type of world order. They are permanent states formed by nations, and the ideology of nationalism regards this as the only legitimate road to state formation. Although linguistic and cultural nationalism influenced past nationalist movements, all existing nation-states are ethno-national. They are trans-generational communities consisting of one or more ethnic groups (titular nation and national minorities). Together these states hold all habitable territory, as contiguous national territories, constituting a nationalist world order.
no existence rights for nations
No nation has a right to exist. There is no ethical basis for such claims, they are simply political demands of nationalist movements. Most nations which ever existed on this planet have disappeared anyway, and there is nothing wrong with that. There is no reason why the currently existing nations should remain in existence for ever, and that applies equally to the nation-states.
alternative geopolitical orders exist
The world order of nation-states is only one possible world order. Alternatives exist, which are fundamentally different - a global caliphate, for instance. In Europe, alternative geopolitical models for the continent are relatively well known - Europe of the Nation States, Europe of the Regions, Europe of the Peoples, and a single Euro-state. Other continental-scale models will soon develop on other continents.
The present geopolitical order in Europe does not have any specific priority over the alternatives. It is legitimate to seek its replacement, which inevitably means the dissolution of the existing nation-states. Nation-states usually prohibit attempts to dissolve them - the crime of treason. Again, there is no ethical basis for this prohibition, and no reason to take it seriously. A united Europe implies that the population of the nation-states commit treason, en masse.
borders are not logical
The present borders of the nation-states in Europe are the result of a long historical process. Few of them were decided peacefully. Many factors influenced the present borders: dynastic rivalry, great-power rivalry, success or failure of secessionist movements, religious allegiance of the population or the rulers, and geopolitical advantages for third parties. Once established, the national borders (or claimed borders) are 'sacred' for nationalists, so it is difficult to change them.
However, one factor was clearly absent in determining the borders: administrative logic. If Europe had no nationalist ideology, and it was divided into new units for administration and regional policy, few of the present borders would survive. Only island nations, such as Iceland, have logical borders, in this sense. Many national borders are absurd, and the abolition of the nation is justified on that ground alone. Hundreds of divided villages and towns in Europe are symbols of this nationalist absurdity. It illustrates the hypocrisy of the nationalists, who found division unacceptable in Berlin, but sacred in Putte or Mouscron or Gorizia. The defeat of the nationalists - and of the nation-states they established - is a precondition for a good administrative and regional structure in Europe.
nation-states are ethnic entities
The only logic of the existing nation-states, - the only reason why a state with these borders has been formed - is the nationalist ideology itself. The states are formed by ethno-national groups, which call themselves 'nations' or 'peoples', and the national territory is defined as their 'homeland'. No other justification for their existence is given: nationalists think this model is self-evident and self-justifying. It is not.
All existing nation-states in Europe use the same central definition of the nation: biological descent from the past inhabitants of the territory. They differ in the degree to which they limit membership: Germany grants citizenship to 'descendants' of people who emigrated centuries ago, before a German state existed. Usually, descendants of emigrants are considered to have lost their nationality, in the second or third generation. However, in the 'opposite direction' there is no limit in time for the claimed ancestors. Claims to the national territory may be based on claimed descent from a population group recorded on the territory 1800 years ago (Romania) or 2600 years ago (Israel). Statistically, the present inhabitants can not correspond to the 'descendants' - unless the territory was sealed, for all that time. Since the number of ancestors of each person doubles per generation, and since there have been large-scale migrations, almost everyone in Europe has some Dacian and Davidian-Jewish ancestors. Most European nation-states trace the ancestral group and its homeland back to the late Middle Ages. Even then, most of the European population must have some ancestors in each 'homeland'. No nation-state grants citizenship to all these 'descendants', so why should some of the descendants get a national territory?
Within the present generation, all nation-states transfer citizenship on biological grounds. If a child is born on the national territory, and its parents belong to the nation, then it belongs to the nation. Different nations have different rules for other circumstances, for instance if one parent is non-national, or if the child is born outside the nation. However, all nation-states transfer citizenship by parentage, and almost all citizens of nation-states acquired their citizenship in that way. Most treat adopted children on the same basis as natural children, but in no case can a childless citizen give a gift of citizenship to non-citizens (which might save their lives if they were starving). Immigrants without citizen parents can acquire citizenship by naturalisation, but the numbers are relatively small, and the conditions are often restrictive. Biological descent will continue to be the prime route to citizenship. No non-national can ever claim citizenship as a right, it is always 'granted' - as a privilege.
Nations define themselves through ancestry and biological descent: inevitably, racism and discrimination are integral to the nation-state. By definition, a nation-state is for one nation, or one cluster of national groups. By definition, membership of the nation is restricted. If immigration to a state was completely free, and citizenship automatic for all immigrants, that would not be a nation-state, in the nationalist view. Most nations have restricted ideas about who belongs to the nation, and derive their citizenship policies from that narrow definition. The child's inheritance of citizenship excludes, by definition, other persons - on biological grounds. Nation-states do treat citizenship as if it was a genetic superiority of a population group. Their policies on this issue are racist in the original 19th-century sense, implying a belief in racial superiority. Zionism is racist is this sense, because it implies that Jews possess a quasi-genetic superior quality, namely the right to live in Israel. But Palestinian nationalists say exactly the same about Palestinians, and Irish nationalists about the Irish, and French nationalists about the French, and German nationalists about the Germans. Zionists are racists, but they have company: all nationalists are racists.
an obsession with the past
Nation-states are by definition oriented to the past. The nation is a transgenerational community, but only the past generations can serve as a cultural reference. No-one knows where their future descendants will live, or what culture they will have. But they do know about their ancestors, their ancestral homeland, and and their ancestral culture - even if only a mythological version. By default, the past culture becomes the national culture. Preservation and protection are typical of the nation-state, innovation is not. This obsession with the past might be harmless in an individual, but nationalists make it a function of state - and so the nation-state becomes an obstacle to innovation. That is a prime justification for its abolition.
alternative use of territory blocked
In more abstract ethical terms, nation-states are wrong because they exclude alternative uses of the national territory - often better uses. Nation-states have no ethical goal other than their own existence, and they use the national territory for that purpose alone. The national culture does include certain values, and these are 'impressed' on the territory of the nation-state. But any value, which is not a national value, is allocated no territory. In a world order of nation-states, the only values and societies are those of the nations. There is no way in which a non-national form of society can be allocated sovereign territory. Equally there is no way in which a value can take effect, if it conflicts with the values of the existing nations. Since no nation-state has 'equality of income' as a value, there is no income-egalitarian society on earth.
Although this may be an abstract moral defect of nation-states, it cannot be peacefully rectified. Since nations jointly hold all habitable territory, any territory for new states would be taken from existing states. And since they all consider their territory 'sacred' and 'inviolable', such a non-ethnic secession would be resisted by armed force.
negation of utopian states
The fact that nation-states block alternative use of territory - and territory is not infinite - indicates a contra-utopian element in nationalist ideology. Nationalists - meaning all those who support the existence of nation-states - are conservative and past-oriented. The national populations already have a culture, by definition, and don't need a new one. So although it does not get much attention in nationalism theory, or in the propaganda of nationalists themselves, nationalism is inherently hostile to experiment and innovation. (If there is any large-scale social innovation in a nation-state, it is usually driven by rivalry with other nations, or wartime necessity). The creation of utopian states in Europe, or any form of innovative sovereign zone, requires the disappearance of the nation-state.