In my case, it was simply the need for Ambient/Fantasy types of sounds, which there are never that many off, in the factory sounds of a machine... I simply needed them to make my own music... so you could say that my approach was predetermined, when it came to the genre, but the actual sounds themselves was a result of experimenting with the synth engine... i never decided in advance how many of each category I wanted, though I understood the need to have at least a good mix... It probably depend on what you're aiming for... a specialized bank with roughly the same type of sounds, or a mixed back of tricks.
So with that as a guideline, I simply just messed around with the synth engine, until something came up that I found nice... it was only in the end phase of the bank design, that I went for a specific type of sound, simply to try and even out the different types a bit.
Personally I'm not very good at planning a lot of stuff ahead of time... what ticks my clock is the fun of sound design, and I need to feel an urge to try something specific if I am to get a good result out of it, or I will simply cut corners because i get bored with it... telling myslef that "Now I need to make 128 sounds with only PADS" would be certain kill... it will be fun for the first 16 sounds, then I'll be dying to create something else
I see my soundbank as a work of art, rather than a product made to please anyone... that's the sheer truth actually... it was made for me, not any of you, and the only reason I decided to sell it, was because so many asked for it actually.