Tying movement speed to dex is a bad idea. Firstly, it makes it so that everything must be on a board of some kind, with precise measuring that actually matters. That's only a little bit of extra complexity in Roll20, but it would be very inconvienent for PbP, as you'd need to post a map and be very clear about how movement works. Also, if a player has very high dex and a ranged weapon, they could pretty much become immune to low speed enemies given enough room. Shoot, run thirty five feet back. Monster chases thirty feet after you. Repeat. Wouldn't be completely OP, as some monsters are smart, but those entrance hall zombies that Julian killed? They seemed pretty stupid.
I already sent you the Oro system modification thing, which made dex more valuable in some cases, but for something applicable to NuER, how about a dodge action? You can choose to not attack, and instead just dodge. If you successfully make a save action, the next otherwise-successful attack doesn't hit you. Since you have to forgo an attack, it isn't OP, but it also doesn't require a special formula to calculate success, and is being rolled instead of an attack, so it doesn't even add bloat!
Alternatively, or additionally, let people take two movement actions instead of a move and attack, if they pass a dex save. Again, no added rolls since they're forgoing an attack, and it could be quite useful for kiting.
Edit: Pan has a fair point about adding in crunch. Adding die sizes to ambush/impaired calculations means that you'd either need to look at a table whenever someone attacks under those circumstances, or list both values next to normal damage on character sheets, for easy reference. Could be better to just go with the traditional RPG standard, and double/halve damage. The only problem with that is very high damage stuff like Julian's 5d6 psi damage, which does an average of 17.5 damage, which means it would still one shot basically anything on an ambush... Could just cap psi damage. Or make it d6->d8->d10->d12->d20 d14, 'cause why the hell should we limit ourselves in this age?.