Waiting in the wings

If someone or something is waiting in the wings, they are not yet active or important, but are ready or likely to be so soon.

For example:

The team has several talented young players waiting in the wings.

Some police were in the wings in case of trouble at the rally.

There are at least a dozen young managers waiting in the wings for Harold to retire. 

This expression alludes to the theatre, where a player waits in the wings or backstage area, unseen by the audience, for his or her turn to come on stage.

http://dictionary.cambridge.org/dictionary/british/wait-in-the-wings

http://dictionary.reference.com/browse/in+the+wings