Come promotion to the promised land Edgy will get a brand new set of private boxes to play in in the top tier of the IP. That will increase the capacity to approx 24,000 which is probably enough as long term I think we'd struggle to sell out if we stayed in the Prem as long as Stoke have. The issues with the IP stem back to it being part of the original plan for that stand to be built when we reach the Prem so the stand we have today is a scaled down version of the original idea.
If, however, the demand was there for a larger ground long term then "safe standing" is one option - might only add a thousand or so to the capacity though.
Filling in the corners would cost too much as the floodlight towers would need some serious modification - would add approx 5,000 to the capacity but would turn Deepdale into a "soulless bowl" of which there are enough already.
Given that Deepdale is in the middle of a residental area the only stand that could have an extra tier built on it would be the Finney, at maybe 3,000 extra seats. Again though not likely to happen and it's highly unlikely that there'll ever be an attendance at Deepdale above 25,000 ever again. Although in reality there's only be a few seasons post-war when we ever averaged over 25,000 and that was when we had the greatest ever English player and virtually every club was experiencing record attendances.