So Blizzard stated that Garrisons will have Multiplyers functions on future patches.
Make up your own
-Pool Party: A pool that if you stay inside for 1 min you get a one hour buff of +50 stamina
-Library: + 50 intelect if you spend 1min reading books on the library
-Profession Vendors: vendors that sell unique materials only available on your garrison. People on trade chat requesting /invites for your garrison to be able to buy the items.
-Arena: Some kind of cool place to have /duels on your garrison.
-Garrison VS Garrison: Blizz said there is no plans for this...but they'll think about it xD
-Gym: Lift weights for 1min for +50 strenght or + 50 agility on the jump rope.
-Master bedroom: For......RP servers, if you know what i mean.
I hope Garrisons will have crazy multiplayer stuff on the future ^^