The rescue plan for Scotland, otherwise known as Devo Max, has of course implications that extends well beyond Scotland. All the other regions of the UK will be affected. One assumes that if Scotland gets new powers, wont Wales and Northern Ireland be entitled to those too? In particular powers relating to tax and spending decisions.
Consider the position of Northern Ireland, it shares a land border with the Eurozone country of Ireland. Now while cross border trade is an important part of the NI economy, its difficult for NI companies to compete with Irish one when taxes south of the border are much lower, in particular corporation tax. So I suspect NI wouldnt mind being allowed to adjust its corporation tax rates to match those of the Republic.
Of course that would leave a deficit in tax revenue that would have to be made up somewhere else. And keep in mind that NI already receives more money under the Barnett formula than any other region of the UK. Inevitably the English will start to worry about being the ones left holding the cheque not to mention the old West Lothian Question that needs resolving.
I recall reading in the London Evening standard (not that I normally pay much attention to what they say, but this was a rare moment of clarity!) an article, during the referendum debate which pointed out the two regions of the UK which have done particularly well since the Blair governments policy of devolution, were London (with it Mayor) and Scotland. The paper argued that having a vocal local government clearly helps a region get a better deal, so presumably an English Parliament, would be a solution.
Of course its not quite that simple. After all, Wales and NI also have a parliament, yet they dont necessarily dont get everything they ask for. London and Scotland have the advantage of holding certain strategic assets that Westminster wants to retain, the money (in London) and the oil (in Scotland). Both of which of course also net a considerable part of the UKs tax revenue. If Wales threatened a referendum on independence, would they have promises lavished on them? Probably not.
Also the costs of running rural services in parts of Scotland, Wales and NI are much higher than in England. And an overcrowded London has a large list of infrastructure projects that are needed to keep the city functioning. In short, theres no guarantee that further democratisation of England would mean England getting a fairer deal, as that largely depends on ones definition of the term fair. Certainly if oil revenues and the earnings of the city of London were excluded, I suspect much of England would find itself worse off.
But in order to avoid any English parliament becoming some sort of talking shop, where has-been & washed-out politicians discuss the route of the new A5 bypass, it would need to be given serious legislative powers. In other words the powers to tax and spend. That opens a whole new can of worms. Not least because, for example, it would relocate much of the debate in this current parliament about austerity, from Westminster to the regions. Parliament effectively reduced to making some limited decisions about foreign policy, defence and some of the broader issues on health care or social welfare policy.
Also such a policy would trust the regions to be capable of managing their own finances, given that in the event of things going tits up it would be Westminster who has to bail the regions out. It would also trust them not do something incredibly stupid (bridge to nowhere sort of stuff). Now while in some Federalist countries, notably Germany, this system has worked well (as the states of Germany would sooner eat cat food for a few decades than ask Berlin to bail them out!). However in other countries, notably Spain, it has been a disaster.
And perhaps more fundamentally, do the English want this? Recall that Tony Blair tried to have a new parliament in Northern England and Cameron has promoted directly elected Mayors. In both cases such policies were largely rejected by the public.