For more information on garrison progression, see my other write-up -- Detailed Garrison Progression For WoD.
Main garrison buildings collectors will need to build (medium plots):
- Barn: Pygmy Cow comes from this building once you upgrade the Barn Level 3. Upgrading to level 3 requires an achievement to unlock the blueprints. Just start/complete enough work orders at the Barn and you'll complete the achievement.
- Inn (A) or Tavern (H): NPCs will visit each day with a daily quest. Keep an eye out for two particular NPCs, Mylune and Cowled Ranger. They have daily quests that reward an Everbloom Peachick and Sentinel's Companion.
- Trading Post: Unlocks a new faction at Level 2, Shattari Defense for alliance, Laughing Skull for horde. Currently only the horde has a pet (Bone Wasp) that's purchasable from the new faction, but an alliance one might be added too.
Profession buildings collectors will be interested in (small plots):
- Tailoring Emporium: If you have tailoring and build this building, you'll gain access to a vendor that sells Draenor recipes. You'll need to craft Secret of Draenor Tailoring (one day cooldown) to use as currency to purchase the recipe for Elekk Plushie.If you don't have the profession that's associated with the building, you won't be able to purchase anything from the vendor.
- Engineering Works: If you have engineering and build this building, you'll gain access to a vendor that sells Draenor recipes. Each pet schematic costs one Secret of Draenor Engineering (one day cooldown). Non-engineers can build this building to craft the pets, but must first upgrade the plot to level three.
- The Forge: If you have blacksmithing and build this building, you'll gain access to a vendor that sells Draenor recipes. You'll need to craft Secret of Draenor Blacksmithing (one day cooldown) to use as currency to purchase the recipe for the Soul of the Forge.
Edit: According to @perculia, you can also purchase the recipes to craft the pets from Ashran vendors.
Miscellaneous buildings that you automatically get as you upgrade your garrison:
- Menagerie (has its own plot): This building is a given since it's all about pets. :P Tt's unlockable at level 100 (requires garrison Town Hall level 3).
- Herb Garden (has its own plot): Herbalists and non-herbalists can loot the pet from Disturbed Podlings that ocassionally spawn when looting a node in this plot. The podlings can also spawn out in the world where only players with the profession can gather herbs.
- Mines (has its own plot): Miners and non-miners can loot the pet from Goren Protectors that ocassionally spawn when looting a node in this plot. The goren can also spawn out in the world where only players with the profession can gather ore.
- Fishing Shack (has its own plot): Two pets will be purchasable from Nat Pagle (with enough reputation) but max fishing will likely be required.