Kalph, you being serious that Source would be a downgrade considering how old the engine is? People keep saying Badspot has modified it to the extent of it being its own engine but idk if that's true.
Source doesn't even a native modding system; it's a joke of an engine.
Torque 3D since its now free, open source.
Oh wow, no no right there. Could that be added/coded by someone or is tricky or impossible? Doesn't L4D2 have addons, well they have maps basically.