I consider these barebones:
- User x user trade system, along with a way for users to sell items on a market be it a market house, user shops, or guild traders, and as well a way for users to list items in a public market space they are seeking offers/specific things for.
- Pet interactivity such as: stats, or dressup, or directly interacting with player (feeding fave foods gives "I love hummus!" or going to a certain location "It's really hot here, can we go to [other city]?", or "Don't forget to play [daily game/activity] with me today!"), just a way to make pets feel more alive and 'yours'.
- A way for users to make their pets their characters. Wajas and Flight Rising have on pet's page text boxes for customization, Subeta and Neopets one can use code to customize a pet's page, and Chicken Smoothie has a forum-focused way for users to create characters, draw, and use them.
- A collection system, of some kind. Most popular seem to be tcg, plushies, parody/reference items, books, and desserts.
- A map with multiple biomes with region specific activities and npcs so users have a reason to not only visit different areas, but develop favourite areas that could be used down the line for events and plots.
- Achievements of a good variety.
Barebone+:
- In world pets, such as petpet/minion/buddy/etc. They are also popular for users to want to collect also.
- A way for users to form groups, such as Neopets abandoned guilds or the better version, Flight Rising's Flights.
Not barebones, but seems to be split when it comes to if a vps will have it:
- Arenas/battledomes. Seems 50/50 if a vps will have them, and many big vps don't have it (Chicken Smoothie, Wajas, come to mind)
- Garbage npc, to cut down on users flooding the marketplace with items the community considers 'junk/worthless'. Some communities can avoid these npcs, but most sites need a garbage collector npc who offers rewards/points for unwanted pets or items. I would say, if this is a pet breeding sim, you WILL eventually need such an npc so that users can "send off/release" unwanted pets so that your pet market isn't cluttered - many breeding sims have learned this the hard way. If it's a more traditional pet site, you can probably get away with it, and if it's a collec-a-thon like Chicken Smoothie you won't need it since there should be no reason for users to want to get rid of items/pets since there is no pet/inventory limit in true collect-a-thons ((if there is, it could create an issues of rarity fixing in those types of games)).