Dont. Let it die.
But if you were to revive it, I guess the main thing is to bring back what its lost: have studios drop their biggest announcements, console/game presentations in live conferences during the show, as opposed to having them do a direct on their own. Conferences should be shorter and to the point as well, sometimes they go for hours and present so little. Make the studios and games themselves be the focus as opposed to tournaments, media personalities, etc, etc. Booths and game tryouts are just a bonus, but the presentations of new games/consoles is always the main draw IMO.
But that'd depend on the studios themselves and some (like Sony) seem to have an issue with how E3 handles their logistic.
Alternatively, fuse it with another event. Make it part of Comic-Con perhaps. But I know that idea will not get a lot of traction.
So again, let it die. Lets not put it in a pedestal.