We all know the AI needs to be tweaked, and countless suggestions have been poured in in this and many other threads.
One of Darvin'3's points is giving a purpose to backstabbing.
I think drawing from the War table game series is one good option. we have to draw 3 mission cards at the beginning of the game.Likewise, in Sins we could have a random set of conditions for a player to achieve its 'personal victory condition'. A start-game option could be configurable to change the game difficulty, by giving 0-10 conditions or winning the game. Default number of conditions would be 3. ALL conditions would have to be met for the game to be won. As a twist, a 'sudden death' mode could be in so that if a mission becomes unachievable, that players instantly loses.
Those missions could have varying difficulties, and the map itself would change the difficulty. Some possibilities:
Build at least N starbases.
Control all ice planets. (same for earth/volcanic/desert)
Control all resource asteroids on neutral gravwells.
Destroy the pirates. (unachievable if someone else does it)
Destroy faction X. (unachievable if someone else does it)
Make all advanced tech pacts.
Ally with faction Y [or list of factions] with at least XX%. (at least 100%)
Control all phase lanes to the sun/all suns.
Colonize planets [planet list here].
Do not kill any opposing faction.
Kill at least N opposing factions.
Control at least N planets.
Build starbases on all neutral gravwells.
Control all artifacts.
Build a trade route generating at least X money (before fleet upkeep discount).
Build a trade route with at least X nodes.
Have at least N planets generating M culture each.
Be the first to colonize planet X/the star.
Raze all capital planets but your own.
Build P superweapons.
Max out fleet capacity and use all slots.
Play without capital ships for at least 80% of the game, and for at least an hour.
Play with only caps, scouts and colonizers for at least 80% of the game, and for at least an hour.
I don't know of anyone which is against the suggestion of having a victory condition of at most N allied players, which I am also in favor of.
Other good suggestions have been placed as well, such as the wonder victory like AoE had / ascension from GalCiv.
Other good options would be economic victory: having the strongest economy AND banking XXX amount of money. Possibly a cap on stored resources and a 'storage bank' logistic structure (and civilian tech) could be introduced just for this purpose.
A diplomatic victory would involve making peace with everyone, more than 100% relationship with everyone for at least X time, and maybe something else, like not having killed any faction.
A domination victory would involve dominating a great portion of planets, like 80%.
A cultural victory would also be fitting to match sid meier's civilization.