me and my friend were thinking about the 40k universe, and we thought that the eventual winners would b 'nids and necrons.
both have no concept of alliance, or dicrimination, or politics. They would not bother each other bcos the 'crons kill to feed the c'tan and the nids kill to feed themselves. u can't eat metal, and the nids dont have souls as such. neither would retreat, neither would make peace.
the nids r basically aliens from Alien, and we kno how devastating they would b if they got to earth. if a single element of a hive fleet got ne where near terra, the imperials would b screwed. also, the orks would eventually turn on each other: they're on the brink of that now, the imperials & tau would eventually kill each other bcos they're like that. the 'crons and 'nids would eventually wipe out all other life in the galaxy.
when this happened, the nids would eat each other, and the crons would go underground again. so the crons, due to their ultimate staying power, would win, wait a few million years, then start again. teh cros r the only things that r not alive, and so would not die out as all species eventually do.
these r just our thoughts, but they seem to make sense...
just say if u have a problem with this, or agree!