"you are overcomplicating this"
I would put it in the 3rd person or include myself in the problem, and I would apologise at the start for saying something negative, so I would say for example:
"Sorry, but I think we're overcomplicating this, what do we think about the following idea..."
I've found that works fantastically because I'm sort of saying I'm wrong or have caused an issue (I haven't) and they're included in my suggested solution (they're not really) so it makes a great way to change peoples minds (if you don't mind pretending you're having a bad idea too and giving them credit for yours).
I apply this to everything and it works great. You get a lot of people taking credit for your good ideas, but I don't mind if it means the solution is better.