Feel free to open my map
Anime Game Altered Melee (AGAM) v1.5 to see how I done it.
At the game start, it immediately created new hidden taverns under the original taverns in my map. Once you selected the tavern, you can see the dummy units Next Page and Previous Page buttons that is immediately removed once purchased by any players, and then selecting the hidden Tavern(s) with the new heroes where the tavern is located.
Most of the trigger for this is located under the Tavern Pages folder in Trigger Editor. Under Melee Initialization trigger I turn on the Neutral Taverns and Neutral Taverns Locations triggers. The tavern units used is under the Neutral Passive - Buildings of Object Editor.