Cult Design Help

I've got a player who's interested in joining a cult. They've got no strong opinions, but I was trying to brainstorm a bit. I want weirdness, like communication outside of meetings only through Meteoromancy or Ichthyomancy. I don't know goals or appropriate flaws. I've got a few initiation virtues I want to include in the cult as minimums.additional Mastery specialties (not a virtue, but still)
Potent Magic
Immunity to Sleep (interpreted to permit overtime rather than extra seasons)
The Great Elixir

What form of potent magic?

And do you want to use the Great Elixir by the standard rules, which requires the Lesser Elixir and Alchemy, or are you looking at making a variant?

Haven't decided, but the personal goal is to permit research/development of potent spells, which will happen regardless of Potent Magic flavour. As for the Great Elixir, the final effect desired is immortality, so a variant is fine as the end result (immortal magus) is more important than the path.

I think you definitely need to work on nailing down a theme for this cult - I'm not seeing one arise naturally from the abilities that you mention.

Anyway. Great Elixir or a variant thereof is probably the best route to immortality within a magus' own body; Ascension to the Hall of Heroes for a more spiritual and theistic immortality (and Living Ghost for a trap immortality). Or, if the cult has a Magic Realm theme, they could bring immortality through the simple expedient of Transformations (though that takes a lot of adventure in the Magic Realm).

It is important to design initiation paths to virtues that are optimal to the cult, and not the initiate. Nearly every cult in TMRE has some suboptimal virtues on the path to ultimate power.

If you want to use Immunity to Sleep as a cult virtue, perhaps the cult was originally founded to oppose the Volshebnii Mechtatli (from TMRE, ch. 12), around the time of the Sharp Awakening, but has drifted in purpose somewhat (toward the immortality stuff). The Art of Memory virtue and the ability to use their memory palaces as lures/traps for their foes somehow might also be considered.