Can't "have an idea to" be used in two ways:
A-To express a scheme, a plan, a method:
(1-I have an idea to launch this new product.
Meaning the same as:
2-I have an idea for launching this new product.)
B-To have as an intention or half-intention:
(3-I have an idea to slap him in the face.
4-I want to slap him in the face.
I don't think 3 is clear as to whether I'll do it or not
B. To feel inclined to do something: have a good mind, have half a mind
1. I have an idea to slap him in the face. (Not OK)
2. I have a good mind to slap him in the face. (OK)
3. I have half a mind to slap him in the face. (OK)