The thing is about cash shops is that they can be really well done (DDO, LOTRO, GW2, RIFT) or they can be really shitty and detrimental to your game (TOR, EQ2 and many others). In my opinion the perfect cash shop is something like this:
Assuming the basic game is free, the cash shop should contain
Cosmetic Items, Mount Skins, etc.
Extra Bank Slots, Bag Slots (GW2 does this real well) which does not include ones you have to unlock with in-game currency.
XP Boosters (like 10% boost), GW2 does this real well again. Most of the time you don't need them, and you get them as drops/rewards so you don't even have to buy them. I actually saved them up and used them at once and never bought one of them during my time.
Vanity Pets
Character Slots
Weapon and Armor Skins if the game is designed for it (transmog or skins like TERA) but I would allow for the more badass looking skins come from dungeons so people know you're awesome at the game then releasing those badass skins 1-2 years later with different color schemes so the original are still unique.
Now, you could also do what TOR and other games do. You can sub to the game for 15$/mo and gain a set amount of currency per month. You also have most of the unlocks (inventory/bank space etc.) and character slots.
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I'm probably missing some things. The one thing I really hate or sometimes love are those lockboxes that can only be purchased with cash shop keys. In NWN the keys were expensive and the boxes yielded crap. In GW2 the keys were cheap but also dropped in-game as well. They yielded crafting stuff and other random gear and were a nice little bonus.