Wow, so many clues fulfilled! I've updated the OP.

That just leaves us with...

• X being used for a variable it’s never been used for before
• a 6/6 artifact creature that costs 3
• “different converted mana cost X or less”
• Leviathan Crab
• Shapeshifter Rogue
• Hydra Horror

I still think that first one might just be the party mechanic, but we'll see. I'm still trying to parse exactly how "different converted mana cost X or less" fits into a sentence. Maybe the Shapeshifter Rogue can copy something new whenever it hits an opponent. Or will it tie into the rogue mill archetype?

Is what I would guess.
Well, probably involving creatures and maybe involving destroying instead, but you get the idea.

Hydra Horror:

"different converted mana cost X or less"

Not sure how I feel about this. At X=1 or 2 it's pretty inefficient retrieval, at X=3-5 I prefer Command the Dreadhorde, and at X=6 I prefer Rise. But this can be a land which is pretty useful. I think ultimately I won't seek it out but if I do happen to pull one it's replacing a basic land or Witch's Cottage.

the land not being legendary with that name is uncomfortable

Nice! That just leaves us with "a 6/6 artifact creature that costs 3" and the Leviathan Crab!

We got the 6/6! One more!

https://scryfall.com/card/znr/239/total ... awa-dragon

might be the card that uses X for something it's never been used before or w/e

That dragon is nuts for several reasons, but the main thing is that it basically ignores commander tax. It is always worth the investment for casting it again. Oh, and also the fact that it can copy any kicked spell, not just instants and sorceries.

The only usage of X on that card is identical to how X is used on Gyrus, Waker of Corpses.

Damn I didnt know he existed

And with that, we're done! Updated the OP with all the answers.

Won't lie, I'm disappointed.
Partly because I wasn't expecting it to be legendary, partly because I feel it's overcomplicated.

Is there a Fling based on toughness?

Let's see, looking through Scryfall, you have Sheltering Word to gain 17 life and Entrapment Maneuver to make 17 1/1's, but nothing that directly hurts your opponent. (There are actually lots of ways to gain life based on toughness, or to make yourself lose life equal to toughness, but that's not very useful.)

This goes great with Doran effects, but sadly for Commander, it doesn't fit into Doran's colors, and it doesn't have defender for Arcades, the Strategist.

Edit: Another search reveals Fruit of the First Tree to draw 17 cards.

So, Charix is the best when you have exactly eight islands, because it makes each activation big, and you have three different modes. Which means playing a ninth island would make it worse, which is a bit counterintuitive.

8 is probably on average a very solid number, but "best" is very contextual. You might want 6 (or 12) just so you can hit for a lot without having it die to a 3-4 damage burn spell.
But yes, you might considering running some number of non-basics if you want to helm this guy in EDH.