The point of national currencies is that the currency's valuation reflects the nation's economy. When you try to tie together countries that are too different, such as Spain and the UK, you risk collapsing the entire thing because Spain's economy ought to be valued far below the UK's. SO, basically that wouldn't be feasible. But, for example, if Mexico ever puts down the cartels, there could potentially be a US-Canada-Mexico currency.
Not that you, OP, give a shit about any of this; you are just waiting for people to say it is a good idea so you can use it as proof of the rise of the antichrist or some such BS.