OpenRA

I couldn't find a way in the documentation, but there ARE 107 mentions of "trait" in the document alone so maybe I just missed it.
Basically, I'd like to prevent the construction of grenadiers and rocket infantry until later, and the player isn't allowed to construct a base, so they can't unlock it by themselves. I'd rather have the option invisible until a certain time period, so I've disabled E2 and E3 for now.
On a similar note that I'm not sure belongs here, I also want Nod to only open fire on civilians in their final wave.

You could use an invisible actor as a prerequisite for E2 and E3, then spawn it in lua whenever you please

Is this really the only way? Oh well... I guess I could do that.

If it's purely time based, you could have an untargetable invisible actor off map that slowly trickles down in health, and when it dies, it removes the prerequisite. I think you can require negations of prerequisites. If not, just make that first invisible actor spawn an actor upon death that provides the desired prerequisite.