Take the CG universe. Throw in Tiberium, the Brotherhood of Nod, and, of course, the Prophet, Kane, Himself. Lean back and watch as everyone's (Charles, Schneizel, Lelouch) plans go horribly fubar. GDI optional.
Normally yes, except that it's slow as hell, and there are effective countermeasures available.
Besides, it's character-driven conflict I'm looking for, not yet another end-of-the-world plot. The Tiberium was simply meant to serve it's usual role as Applied Phlebotium, nothing more. The CG-settings' Sakuradite could serve in a similar role, but doesn't have the same synergy with Kane and the BoN.
If slow, you mean it alters the biosphere of an entire planet in under a century or so.
On the individual level, the stuff can corrupt and consume a human in hours. Tanks and other armored structures are also vunrable to it's ability to consume arbitary material and make more of itself.
I am aware of what Tiberium does, which is why I think that it just doesn't cut it as far as a grey goo plot device goes; there simply are more effective alternatives out there (like the ever-popular nanits).
It does work as a threat on a personal level though, yes.
