Indeed but they either entered the EU before the Euro and negotiated an opt out or they're in the process of adopting the Euro (albeit reluctantly in some cases

Indeed but they either entered the EU before the Euro and negotiated an opt out or they're in the process of adopting the Euro (albeit reluctantly in some cases). From the EU's perspective, in order for them to go against the anti-separatists such as Spain, they'd need to have a big carrot. My guess is that full EU i.e. Euro, no opt outs and no rebate would be sufficient as it's fully in keeping with the "ever closer union" agenda. I can't see anything less being sufficiently beneficial to the EU to make the necessary compromises worth it.