You do not need every current EU member to agree to this. You just need enough for it to make sense. If for example you have German, France, Spain, Portugal and Belgium, which are the countries more or less in favor of federalism at the moment, it is enough for most of the effects of scale to work. Thats like half the population of the EU and with a large share of the GDP. Maybe some of the eastern members join as well, which would mean even more strength.