what increases threat range?

[buddahcjcc]

Lunge Ive seen increases it by 5' at the cost of 2 AC
Reach weapon of course, Large size, so Enlarge Person, anything else?

[mplindustries]

99% of people refer to the numbers on the die that threaten a critical hit when they speak of threat range--for example, the threat range of a rapier is 18-20.

You're talking about the reach at which you threaten. Combat Patrol is pretty sweet for boosting that.

The ultimate, though, in my opinion, is using Plant Shape to become a Quickwood--60' reach? Yes, please.