I'll definitely be curious to see your downtube Nexus shifter prototype.

I realize it can't work exactly like a normal downtube shifter, but it still seems doable.

If you build a Nexus shifter would it work on an Alfine hub?

I'll show my complete ignorance here by asking a stupid question, but what is the reason that almost all of these use twist grip shifters.

Hypothesis #1: they think people like them better. Aaargh, surely not.

Hypothesis #2: the cable travel to change gears is much greater and therefore a bar end shifter doesn't have enough travel and a downtube style shifter would have >360 degrees of travel. I do think that my Alfine Rapid fire shifter seems to require quite a lengthy bit of thumb travel to upshift.

Hypothesis #3: I'm not knowledgeable enough to make an accurate hypothesis.