UT/Quake-likes have been suggested for a while now, particularly free ones such as Xonotic, Warsow, Cube 2, and OpenArena.
EDIT: Back to TF2, alternatively making it a public server (with exceptions made for private events) might work. It'll probably turn to a Valve server with semi-competitive rules that way, but certain concessions for members (eg. reserved slots) are in conideration.