“Encore!” at the theatre

At concerts it is reasonably common for the audience to call for an encore. The performers will then pick up their instruments and perform a little extra piece (as opposed to pop concerts where, bizarrely, the band will save their one big hit for the encore).
This idea is that this practice should be extended into the theatre. If the play was especially good, the audience might call for an encore and the actors would have to carry on the action of the play beyond the point envisaged by the playwright, either through improvisation or by preparing this in advance. For some plays this could be hard; E.g. King Lear - most of the characters in the play are dead by the end, so the encore would have to explore the relationships between the characters in the afterlife. For others, like Uncle Vanya (which I saw last night, by the way), the characters would sit around as winter set in, getting drunk on vodka.
Actually this isn't a very good idea, but you can't deny that it's halfbaked.