Inside moving cockpit view, giving the feeling that you are a person in the ship (Think Metroid prime.) to let you center your camera on any part of the interior you wished, and to shake you about when hit by a scratcher.
Bots that have become semi-organic (kinda like the boss on mars in d3)
Lights that make the gameplay more suspenseful. a feeling of distant terror that would make you want to pause, open the cockpit and breathe.
Weapons that can be upgraded or paired to compliment each other. (like fusion and napalm) and cooling systems for them, so you cannot fire forever. (maybe the AB cooler should cool all energy expenditures.)
New weapons like a homing lazar (I think this would appeal to players who die from the gnats, and not the big brutes) Or weapon mods to let you alter weapons (perhaps the Plasma cannon cannot be paired with other weapons, but can be modified to act like the phoenix of spreadfire cannons.)
I ask only that you say nothing about the old games unless it is for comparison to a new idea for Descent4. I tire of people brooding about how Descent 2 was the best, and how they overstepped it with D3. Zelda would never have made it out of the Game boy unless they had thought of what would make it push the limits for consoles. You have a chance to tell us how to do it up right. Do it here, and feel free to talk about alternatives. The only threat facing the fan base here is lack of imagination. I have posted plenty for you to express your thoughts about. If you don't like an idea, then say so (politely of course), but don't say how D1 or D2 was the best until you can say how it can be redone to fit the present gaming industry.
other than that any idea goes...