So far we've got 7, which means we could perhaps do a core team of 6 and then the one "special" color, or 5 and two (Green and White, etc)
The basic plot idea I've thought of, more or less, is that the Z-Wave passed through a...Nebula, Spatial field, what have you, and was corrupted (since the wave is made up of incalculable energy, time itself doesn't matter, would come back and manifest during present day thanks to nebula.) It would hit this group of normally peaceable scientists, alchemists, sorcerers, perhaps 5 or 6, each with a different field or specialty. Corrupted, they decided to raise armies, and then have one final duel to determine who should rule the universe. One would have chosen to come to Earth at some point, having an army of let's say robot minions (think Putties) bio-droids (monsters) and eventually himself (Rita/Zedd/etc). This would also allow for him to eventually be defeated (If the game lasts that long) and a new one of his brethren to come to earth, even stronger, leading to new powers, zords, etc. The minions and droids would change per each lead villain's speciality, the Alchemists would perhaps be more earth/nature based.
As for "Theme" of the team, my basic thought on that was that an ancient priestess/oracle forsaw this danger, in only vague terms of course, and prayed to the Gods to assist her. Whether they were really the Greek Gods of old or benevolent aliens could be a matter for debate. Answering her prayers, they gave her pieces of jewlrey, each imbued with some of their might, which would transform them into mighty warriors, with powerful weapons and their own mighty beasts (Zords) to commands. They cursed/blessed the priestess w/ eternal life, so that should would remain vigilant to the threat. Each god had several symbols to chose from and what not so would have plenty of options for Zords. The powers themselves would not neccessarily denote whose in charge however (IE, Zeus power would not automatically make the person wielding it in charge.)
Thoughts? Personally I'd be drawn to the Aries "power" myself.