So you want to build a synth that rocks the house right? Getting sufficient information and knowing what you like musically is a good place to start.
The inspiration for this case is a drums centric synth with minimal sequencing. Having lots of controls over each individual drum voice (bass drum, snare, hihats, claps, etc) is also a must. I also want the ability to change the noise source and give them a distinct character. Drum sounds need variety if your going to be jamming for hours on end.
Keeping the melody lines simple is also an important consideration. Being able to turn a few knobs and generate pleasing series of tones really appeals to me. Wish my keyboard playing skills allowed me to just twinkle my fingers musically, but gates and CVs are the next best thing!
Changing the character of the lead and other sounds is also a high priority. To start songs I either pick a drum pattern or a tone/noise to build on. So to be musically inspired I need some way to mangle things up and spark my curiosity. Layering or alternating between different voices to play the same melody or bass line would be nice and to be able to flip a switch to do that is even more so.
What's a rig without some good effects right? But before going too crazy I want to start out with some simple delay type effects. I also want an easy way to smear the sound and change it in unexpected and creative ways. As modular synth output can be unwieldy at times compression in the chain to glue things together would be useful as well.
Hope that was helpful to you. Can't wait to see your synth!