It is always a pleasure to see a new player in the headphone arena!
What makes the perfect headphone you ask? Well, sadly there is no perfect answer, but there are a few things to really look into.
Balance and neutrality are of utmost importance to me. A headphone needs to be able to play all music properly and faithfully!
So, the headphone needs to have an extended low end response, without sounding clouded. So, there should be as little roll off as possible, and as little boost as possible, making the bass realistic, tight, and often lacking in a headphone -deep, 20 hz deep. Often mid bass is simply bumped, and there is severe roll off after ~60 hz. Mids should be involving, and highs should be high and sparkly but again, they should not dictate how my music sounds, the music should.
Transparency and neutrality are invaluable. If you follow this advice, you can be sure your headphone can be universally liked and respected, but not everyones favorite. Word of mouth is key, so if someone can recommend it without liking it best, you won.
Detail and speed second to me, I should be able to hear everything that was recorded when I play a recording - everything. Drum stick clicks, coughs, sliding on a guitar string while changing a chord, background noises. And a quick headphone will always impress too, being able to keep up with music keeps the sound fresh and clear. What better way to make music sound like and involving than presenting the whole, unadulterated sound?
Thirdly, comfort! A headphone should be overly comfy! Avoid using cheap parts, and excessive clamping. The extra touches go a long way for the user, and will only save at most a few bucks per headphone. Leather would seem to appeal to older folks more, while velor or just high quality padding is more than enough. Durability is of course to be considered. Comfort without durability is useless! Being able to use a headphone without discomfort means you are free to be aware of only the sound!
Looks are less important, especially if you are going to keep your prices more reasonable. And the lower the price, the more likely people will buy it! People can get over looks, they can't get over sound or comfort. So as pleasing as pretty headphones are, it really is superfluous as you can't see them when you wear them!
Visceral impact is something to take into account too. It makes the music sound much more believable. While I can't say how this is achieved, I no longer listen to my speakers much since I got my W1000X from audio technica as it satisfies that aspect of music.
Lastly, soundstage should be be what ties everything together in a coherent, fluid way. It should not be overly large as it sounds artificial and thin, it should not be tiny, as this sounds bad and cramped. It should allow everything to get through, image well, and be downright convincing. Choice of housing and how things echo and reverb is absolutely key! Housing can make or break a headphone, especially when close!
Oh! and replaceable pads and also, the cable should allow the end user to re-terminate (not re-cable) it to a 4 pin balance jack, so have 4 wires (with writing indicating what is what if possible)!!! If your headphone is truly capable, people will want to try it through balanced amplification!
Focus on universal competence, matching to individual taste can always be done through an EQ and amp pairing! It should not be done on the headphone end.
Should you follow these guidelines, you are sure to have a hit, peoples varying tastes aside.