There's a different idea ...

[MeyaN]

There's a different idea on/in my mind.
What's on/in your mind?

Kindly tell me whether it should be 'on' or 'in', or both can be used.

 

[teechar]

"On your mind" and "in your mind" are not necessarily interchangeable, and each may be more suitable in a given situation. What context did you have in mind? ;-)