Manchester has different priorities to Maidenhead , Wigan has different priorities to Wycombe, Aberdeen has different priorities to Abersoch, Belfast has different priorities to Bristol and so on.
You use devolution of powers and decentralisation of treasury spending to the those disparate area's so they can concentrate on what there specific needs are. It works, it is happening now in Manchester where this Government much to its credit has given part of the Health budget to Manchester and instated metro Mayors with the power to decide where Manchester's priorities lie because our priorities are different to Maidenheads.
There is no reason apart from political will that model could not be expanded so that the priorities of Madrid, Munich and Milan all had the same as Manchester.
The best way to tackle Nationalism is through Localism, give local people the democratic power to decide what is best for their communities. Of course we could still be separate nations and support England v Estonia or Germany v Greece at sport, but we do that locally now as we support Manchester City v Mansfield. Their is nothing wrong with being proud of your identity, but we all have multiple identities, I am a Salford born, City supporting Mancunian, who lives in Cheadle, I am English, British and why not European too, because I do live in Europe.
The only people who would resent it are outright Nationalists, people who do think they are exceptional and think somehow there identity is more special than somebody else's, but when you break identity down they are not, they are products of their local environment so return power to local environments and they can be proud of there own environment, their local town, their local City, their County, their Country, their Nation and their Continent.
We are all human beings, we thrived as a species because we co-operated with each other, we traded with each other, we exchanged ideas, we told stories, we didn't thrive by being insular, uncooperative , silent and protective.