To which I would add that we already have both Invaders and the Arachnid Omnivoracity. We have ever-increasing, ever-improving 'outside' forces that can't be counter-invaded, only endured or stoppered. And we have suicidally-hostile, wipe-out-all-life, massively out-produce player empire(s) yet technologically slow/stagnant enemies.
Certainly, the 'implacable foe' and/or 'endless horde' is a staple of space opera, and has a place in Aurora. But I agree that spoiler races need to be significantly different from not only what a player can do, but from each other.
Personally, I also think they should be a surprise. That is why we spoiler them after all. I wouldn't want to know all the mechanics behind them (as are laid out above). There are more things in space (and Aurora) than are dreamt of in my empire's philosophy.
I am excited to discover organically the changes and additions Steve has made to the spoiler races, and if he wants to do something like The Forbidden is suggesting, hopefully he'll keep the details quiet until well after I've had my grubby little manipulators on C# Aurora for a while.
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First, though, I want Steve to build the "bug button" he promised me:
http://aurora2.pentarch.org/index.php?topic=9841.msg110045#msg110045