I'm pleased to see that some of my favorites are listed, including some of my mods. Yay!
BTW, Conquest & Ace of Nails both have features that are pretty enough that they might be included. Conquest especially makes use of things like Spark Lightning that are going to show well (see: Tesla Bomb). If you had a mech firing its railguns at the player and showing his shields flash to block it (or something similar), I think that would show of a lot of what the mod is about. Another option is to imitate , with the plasma arc traveling past some enemies and zapping the hell out of them.
Another of mine to consider is Final Fantasy Tactics Quake. It's a very incomplete tech demo, but it may be cool to show off. However, considering we have so many very viable candidates, I'd hate to push out a more complete mod to show off FFTQ.
As for stuff that isn't mine (I'm being way too narcissistic talking about my own stuff only anyway):
If we could show off Rune Quake nicely that would be a good addition. I also agree with showing off Nexuiz as it is Quake1 based, and something to be proud of.

It also displays well. Although, they've moved on to Quake3 I think.
Archmage, if no one's mentioned it yet, though it's an old, old mod. It's still cool to see your hands move and launch a fireball.
It's too bad we have only 6 seconds per mod. Many of these deserved at least double that to really show off. However, too long a video would not be good.
This is a great idea! Cheers!
When my computer inevitably explodes and kills me, my cat inherits everything I own. He may be the only one capable of continuing my work.