Scotland is poorer than England, but they could remain independent, and perhaps create a mini Empire (they wouldn't have enough resources for any big adventures, but wouldn't it be fun if Nova Scotia acctually was Scottish??)
What resources could Scotland use? The shipping and maritime would be decimated. One or the other would have to come to dominate in some form or fashion. I guess what if Scotland defeated/ occupied Ireland first...
Prevent Scotland blowing it's money on a disease infested swamp in Panama and the English passing 'Alien' taxes on Scottish goods and you keep Scotland seperate I'd think.
Whether Scotland can have a sizeable empire is another idea, of course Scotland were hugely involved in creating the British one. But it won't be economically strong unless it can reap the benefits of the Scottish Enlightenment.
Or go back a long way and stop Scotland losing control of what is now Northern England, might create a more balanced situation between the nations.
Stop them losing control? Make them gain control I think you mean.
With this sort of scenario its strange as a lot of people seem to see anything with the name Scotland as inherantly celtic. But for Scotland to be a powerful nation it'd have to leave that part of itself even further behind than IOTL.
Some sort of revived Northumbria (not with that name though- they'd probally keep the Scotland branding) is the best way.