Not "we" but perhaps that is what is missing in the Eurozone, and there lies the problem with the EU.
Before the Euro, we had the beginnings of a single market and we also had the customs union.
Once we had the Euro and added all the new countries it all began to fall down.
Lack of fiscal transfer as discussed.
It is the start of a two-tier Europe and this is where the project will probably crumble.
Without that fiscal transfer, the Euro will fall, and it needs to be at least 10 fold what it currently is, which is something I doubt the wealthier Euro nations will stomach and the non-Euro nations won't want to pay for.
Those outside the Euro would just be more disadvantaged trading with the Eurozone and by being in the CU as the policy would have to be geared towards the Eurozone.
The "ideal model" would probably be for more integration in the Eurozone, but then a loser EFTA style arrangement for the non-Euro nations.