I'm not sure there's necessarily a "right" answer, especially if talking about "Imperializing" a recipe that doesn't have a tradition of being made ultra-big (like making imperial stouts and I-IPAs).
The way that I would approach it is to consider really what I was trying to ultimately accomplish. Thinking about an imperial pilsner, for example. If I were doing something like that, the "pilsner" characteristics I would be looking to maximize would be perhaps the hopping and some of the malt flavor, but I would not want it to be too sweet/cloying or too thick (I'd approach it completely differently than DogFish Head). That would mean that I would *not* bump any specialty malts (not that there would be many), and I'd take steps to maximize the attenuation of what would be a greater amount of base malt. I'd think about low mash temps, longer mash times, the introduction of simple sugars, etc. I'd add a lot more hops and a lot more later hop additions, keeping to classic varieties like Saaz so that it remains somewhat identifiable as "Pilsner." I'd keep the yeast choice as clean as I could, I'd pitch a HUGE starter so as to minimize ester production.
But, those are all steps I would take based on the beer I ultimately envisioned. If I wanted to make an Imperial English Brown, I'd be maximizing some of the fruity notes, probably increasing some of the crystal malts, making it even sweeter than usual.
Philisophically, to me making something "Imperial" means identifying the core characteristic of that beer and then taking that to the n-th degree. For different beers, that key characteristic is different; malty, hoppy, fruity, sweet, etc. So, I think you need to approach this on a beer-by-beer basis.
But, that's just how I would approach this question, philisophically.