I had to dig down for this, but wanted to write somethings down.
1. An indicator light telling you when your button is depressed would be good. I know some people's buttons get stuck, and they prob don't notice til they see smoke coming up from the table where they set their PT down. For me it was my couch, haha. And it would be super cool!
2. re-enforce the end connection, i understand the e-clip may be durable, but it seems to be a single point of failure right now, though i haven't opened mine up to check to see if there is another stress point in place.
3. i see you said you were taking the protege switch over, that is most excellent as it completely alleviates my number 3.
4. more flexible cord, but same gauge? I don't know if that's possible, according to your measurements in your "how to test volts under load" test going from 20 to 24 gauge wire dropped output under load by .3 volts (or something like that, maybe it was closer to .2), thats too much for me. If they make more flexible cable that's the same gauge as the current, then i would say go for it. I don't think they do though. You could alleviate this concer by incorporating a stress point via the swivel end that was mentioned earlier in this thread.
5. Shorter cord, 4.5 feet should do, 3 ft is often too short for someone to be at the computer, and use their vaporizer while standing, 6 ft. is too long for a lot of people's preference, (i often am passing by, and have it ay my computer and want to take a quick vape on the way by), 4.5 feet works great... i tested it, on a 36 inch computer desk, and i am 6'1, so there you go 3 foot was too short, 6 foot was plenty long enough, but creates a cable management issue, 4.5 feet. perfectumundo.
6. maybe offer a resisted switch? I wouldn't want it, but a lot of your die hard 3.7v vapers would love it i am sure. I am not sure if the current form factor of the PT itself would support it, but you could put a resistor in the cable.... though i am not sure if that would become cost prohibitive, haha.
7. body color, some people like the bling.... shiney chrom, some people don't, you could even design for one to be a sleeve that fits over with the adapter and switch off, man that would be sweet.
Thats about it, just some food for thought, i love the current PT as is, its effing sweet.
Gibby