It seems to have become a popular Unionist pastime to devise schemes for slight changes to the devolution settlement, thinking that Scots will mistake them for devo-max and vote No to independence as a consequence.
However, according to the Scottish Social Attitudes Survey (PDF), 32% of Scots agree that "the UK government should make decisions about defence and foreign affairs; the Scottish Parliament should decide everything else" (in addition to the 31% who want all decisions to be made in Scotland). A few cosmetic changes to the status quo are clearly not enough to create a viable alternative to independence.
If we look at public spending in Scotland (the graph on the right), it's clear that more is already spent by Holyrood (the blue bits) than by Westminster (the red bits). To achieve devo-max, the remainder of the "social protection" spending would have to be moved from London to Edinburgh.
Interestingly, the rest of the non-devolved public spending adds up to peanuts (about £8bn), which means that it could all be paid for by VAT (which raised £9347m in 2012-13). As a consequence, all taxation apart from VAT could be devolved to Scotland, and all block grants and other fiscal transfers could be abolished.
There would obviously need to be a federal parliament to deal with foreign affairs, defence and VAT. Because it would have so little to deal with, it could be much smaller than the current House of Commons, and the seats should be allocated according to Penrose's square-root formula, giving Scotland about 18% of the seats, ensuring that Scotland wouldn't get less influence than it would have as an independent country.
In addition to the changes above, we'd need a proper constitution, preventing Westminster from ever rolling back devolution against the wishes of Scotland, and enshrining Scotland's eternal right to self-determination.
There's no reason why all of the above couldn't be signed into law before the referendum to ensure that the Unionists don't suddenly change their mind afterwards.
Sadly, it's probably more likely that pigs will fly. Unionist politicians are showing absolutely no signs that they'll ever agree to something as simple and reasonable as this.