Here's a list of things that I think would provide a great avenue of expansion for not only the cash shop, but also the game.
1. Backpack Slots
This is one of the most profitable sources of revenue for the cash shop in Gw2. People like back items and backpacks. You already have some great looking backpacks a part of various costumes. You should 100% put in a new cosmetic slots that is for a backpack skin. Not only does this give you a WHOLE OTHER desirable reward location for players to go for with content (backpack from ToT, backpack from dungeons, from digging, the options are endless), you can also sell various skins in the shop. They'll cost significantly less development time than a full fledge skin.
2. Mount Armor
This one may be risky because the horse armor memes will be powerful. But perhaps look into selling variations of the upgrade looks of mounts. How your mount changes looks as its upgrade. What if you give players the option to customize that look? It would offer yet another way to offer not only cosmetic in game, but also continue to "profit" off of already made mounts. Of course you dont want to sell it for the same price of the mounts. Its gonna have to be quite a bit cheaper else players will meme on it. The other important thing to keep in mind with this method is that you need to make upgrading your mount less cancer. It's not a fun experience and many players probably forget about it on a daily basis. If you offer upgrade items in the game as potential rewards from content that needs a population boost, would help the situation significantly. You do have seals of endeavor, which maybe the lessons are a "good" drain for them. But I'd be surprised if lessons are popular purchases. Customizable mount armor would + offering riding lessons in game would be better in the long run IMO. Especially since people love factions. As you introduce factions and groups of NPCs in the game, you can release corresponding mount equipment. All the houses of Vvardenfell, each of the 3 factions, different holds in skyrim, religious icons, etc.
3. Houses
There's a lot of good locations a house can be put. More people are asking for medium size houses, and I agree. Most of these locations you can just use pre-exisitng layouts. You can even put multiple ones to allow people to have "neighbors". Here's some great locations for smaller/manner sized locations to allow people to purchase a house
1. Balmora
2. Morthal
3. Orsinium
4. Abah's Landing
5. Kvatch
6. Anvil
7. Vastyr
8. Lilmoth
9. Shimmerene
10. Ald'ruhn (ashlander themed)
11. Suran
12. Various outlaw den locations - What's stopping you guys from putting a house in an outlaw den? Just a door that on the other side is a nice housing area themed in that of the outlaws den location
13. The Hollow City
Some of these locations already have houses with locked doors (Balmora) that you can just convert into purchasable housing...
Vvardenfell is an especially big gold mine in my opinion because of all the nostalgia.