The Colosseum in Rome, of course, is a 2000 year old arena that once held massive events, but is now in a state of disrepair and has not been used as intended for over a thousand years.
What would be necessary to repair/renovate enough of it so that it could be used again for bona-fide theatrical and sporting events? If the floor were restored, I could easily see a basketball game happening there.
By repair/renovate, I mean fixes, modifications, or additions that would preserve the historical integrity of the Colosseum (i.e. no bulldozing it and building a replica). Using modern construction technology, building materials, and modern building fixtures such as electrical power or flush toilets is fine.