what comes with the expansion pack? I've only heard of one big update to XC2 that was purchasable and wasn't Torna, and that was the ... uhh ... Arena I think? With Shulk?

I can't give you a definitive list of what comes with the expansion pass, because some things were free updates for all players, but...

1. Useful items like protocols to transfer blades, a bunch of gold, 60k ether crystals to upgrade Poppi without having to play the virtual video game; minor items like favorite food of certain characters.

2. 20 regular sidequests scattered across the entire story progression, so some quests are available from chapter 2 onwards, others are for the late game.

3. New rare blades with their own sidequests. The base game had 38 rare blades, all updates bring the total to 52. At least half of the additional 14 were part of free updates.

4. The arena with Shulk, Fiora and Elma. Features ~30 challenges. Rewards are customes for various characters.

5. Hard difficulty, easy and custom, and new game plus should all be part of free updates, just like various improvements to map and item sorting functionality.

I'd say that expansion pass stuff is worth no more than $/€10, because it's rather insignificant in the big picture. But obviously Torna is included in the $/€30 price of admission, so the expansion pass is a steal.

OP's link says that it requires Xenoblade Chronicles 2 to even play Torna. Nintendo's site says otherwise. Torna is a standalone game, so it will need a physical card, unless Nintendo is ditching physical cards for its games in the future. But that would be a terrible move on their part.

Update: I phoned Nintendo directly (har har) to ask them what the go was.

The rep I spoke to confessed he didn't know and found it confusing and unclear himself, and said he had to speak to his superiors.

After 5 minutes on hold, he returned to inform me it was "most likely" just a code in a box.

That's not the most convincing declaration I've seen so far,lol.

Seems like even his superior doesn't know  for sure ...

Though, congrats for the courage you stacked to ask them directly !

Thanks man.

And yeah, it is a bit of a bizarre situation when even Nintendo reps seem to be in the dark and can't give a definitive answer.

I thought Nintendo made it clear that Xenoblade Chronicles 2: Torna can is a physical standalone game with a download code for the rest of Xenoblade Chronicle 2's DLC.

Until it is released who knows.

Nintendo has done the code in boxes thing in Japan, this maybe a other case of that. Although given the lush I would think it would have to have some sort of stand alone cartridge. Yet you never know what they consider.

One day this obsession with plastic will end and I will celebrate.

Well, you can always just celebrate now, since you can purchase anything you want digitally anyways. 

I'm going to cancel my pre-order if there is no cartridge... But I will wait for a definitive answer.