Adding a fifth player was essential, although by five players there was already the risk that the game might have ended before you had a turn (and you might have won it!)
The powers. Note: my definition of an essential power is not the most powerfule, but powers that play well, are balanced, and add to the fun of the game, without stultifying game play. That is why Seeker is an essential power, even more so than Virus. On the other hand, I don't like Void, because the game simply becomes less fun when you have less pieces.
Skeptic: you can doubt the other player will win, doubling the rewards or losses. An essential power. Fun to play.
Plant: you can use any power whose system you have a base in. An essential power, and great when playing hidden powers.
Grudge: If you invite people to ally and they don't, you can toss them in the warp. A good addition to the game, if not quite essential, as some player choices became a little limited.
Dictator: Can tell a player where to attack. An essential power, whose "choice limitations" become something to overcome.
Will: Can attack anyone whereever they want. A fairly weak power, but only because we played with the wrong rules for twenty years. We played that you must attack anyone in the system, not only the player in his system. Thus, Will didn't do much.
Crystal: tells allies how many to bring. A fairly weak power, much worse than Grudge or Parasite or even Magnet.
Worm: repoints cone after cards revealed. An essential power.
Magnet: prevents or forces alliances. A little weak, but better than Crystal. Better with more players.
Seeker: Ask any yes or no question every round. An essential power, probably the best power ever. Wonderfully balanced, and enjoyable to play.
Parasite: May ally whenever he wants. The reason than you may have won the game even before you get a turn to play. In our games, there was a gentleman's agreement not to invite allies without intending to win, with the exception of the Parasite or Magnet.