The civitas ( = polis) was founded as a city wherein to avoid the dangers of the louts and barbarians up in them thar hills.
For this it was necessary that the different factions within the civitas agree that 'I won't bash you on the head if you won't bash me on the head'; that they would be civilised and polite. Merely being different would not be a legitimate reason - which sometimes do arise - to bash someone on the head. Legitimate reasons arise mainly to do with someone who is not civilised.
A nearby civitas can, if only because of competition for resources, become a threat as much as barbarians; but when this is avoided it's called 'international peace'.
Nations (= polis (nearly)) are not only defined by geography. Jews and Gypsies are nations without geographic territory.
Nations are not only defined by ethnicity. Different classes are in many ways different nations. (cf. Marx., HG Wells' 'Time Machine).
Nations are not only defined by money. Groups such as (in current, first world terms) homosexuals, drug users, post-consumerists communists and feudalists, (in Cromwellian terms) papists/moslems, witches, levellers, (in colonial terms) the culture of the subordinated population, them goddam, heathen, injun savages, are also nations who deserve civilised treatment if they behave in a civilised way.
The dominant Weltanschauung is consumerism. (And let us not forget what glorious, mind boggling benefits consumerism, especially in its capitalist form, has brought us.)
A political system, in contrast with the electoral systems of current 1st. world countries, could be developed to protect the wishes of several different nations. This might involve the allocation of different zones (countries) to different nations (just think if the United States of America had allocated a few states to them goddam injuns (and if the War of North and South had gone differently)) and by recognising interzonal nations (eg. Gypsies and Jews) to have their own rights and authorities. In making their decisions they would need to take into account data from elections. Data from elections in a first-past-the-post system would not be of much use.
Zone allocation would be the duty of - to speak more precisely and prescriptively about a possible EU constitution - the House of Nations (cf. House of Lords (ie. those who are more noble than the common hordes)), while the House of Parties (cf. House of Commons) would be more concerned with mundane administration.