I consider, that you are mistaken. I suggest it to discuss.
I apologise, but, in my opinion, you are not right. Let's discuss. Write to me in PM, we will talk.
I consider, that the theme is rather interesting. I suggest all to take part in discussion more actively.
I think, that you commit an error. I can defend the position. Write to me in PM, we will communicate.
I apologise, but, in my opinion, you are not right. I am assured. Let's discuss it.