There are two Expansions, each one offering 10 unique items for Vanessa and Pygmalien.
Unlocking this optional content will expand the items available to you during a run of The Bazaar.
Let's take a look at the first few cards revealed in the patch video.
The remaining cards can be viewed in their tooltips below.
Mysteries of the Deep (Vanessa's Expansion)

Mysteries of the Deep appears to be an Aquatic-based Expansion, as teased by the art and two currently known items.
Given Vanessa's vast pool of Aquatic items, there's a lot of potential for some crazy new combos.

Primordial Depth Charge is just one example, giving players an awesome build-around payoff. With a board full of Small Aquatic items, it's possible to give this +8 Multicast (9 casts total) that will set off a chain of Freezes combined with damage-over-time.

Diving Helmet on the other hand is pretty simple on the surface, but has the unique effect of applying the Aquatic tag to adjacent items. Aquatic builds very often care about their tag, so this helps enable better synergy with non-Aquatic items.
Frozen Assets (Pygmalien's Expansion)

Frozen Assets looks to be expanding the Freeze mechanic, as represented by this cool new artwork.
Knowing that Freeze is often used more sparingly (given how powerful it is), it'll be interesting to see how this expands the mechanic.

Snowmobile has the potential to be a very fast and threatening Weapon, assuming you can assemble enough Haste and/or Freeze support. Pyg isn't normally known for Haste, so I would expect the new Freeze items to do more of the heavy lifting.

Aurora Dome is a really interesting counter-Freeze option, which rarely exist in The Bazaar. However, what makes this even more interesting is that it's another Medium-sized Property, and those are quite rare among Pygmalien's items.