What is an antonym of alcove?

There isn't a single perfect antonym for "alcove" as the word describes a specific architectural feature. However, depending on the context, you could use words like:

Opposites in terms of space:

* Open area

* Wide space

* Main room

* Central area

Opposites in terms of purpose:

* Public space (if the alcove is private)

* Main thoroughfare (if the alcove is a secluded nook)

Opposites in terms of size:

* Large room (if the alcove is small)

The best antonym will depend on the specific situation and what you want to emphasize.