There is TONS of material out there if you're willing to look.
Shakespeare is the obvious suggestion.
She Stoops to Conquer by Oliver Goldsmith is a fun comedy that could work.
Throughout history the church has been at various times the greatest supporter and biggest censor of the theatre. An example of church supported theatre would be
morality plays, Everyman being an example.
If you like Celtic flavor, look at the works of
William Butler Yeats and
Lady Augusta Gregory. Both wrote plays inspired by the celtic mythology.
Any of your ancient Greek and Roman works could be adapted.
Look at
Lope de Vega and
Miguel de Cervantesfor Spanish influence.
Also look at
commedia dell'arte. It originated in Italy and spread throughout Europe. Basically, it was a travelling company of stock characters that travelled from town to town doing improvised shows. They had a wagon which they would park in the town square, then the wagon converted into the stage and they would perform for donations.
Another style that could be appropriate is the
melodrama.
Gilbert and Sullivan should be easy to work in, especially The Pirates of Penzance.
Also bear in mind that many of these historical works are a great place to mine ideas for plotlines. I've been toying with the idea of running games based on Henrik Ibsen's
An Enemy of the People, Eugene Ionesco's
Rhinoceros and Stephen Sondheim's
Sweeney Todd (now playing at a movie theatre near you).
Edit: Little Shop of Horrors could also make for a fun adventure. Use the greenvice from MMII for Audrey II.