My thumbnails have drifted from super simple to theatrical. I have settled on the simple side. But even so, they are so hard to judge, and I don't think there is any tool or person out there that knows for sure, except for success or failure. I have steered away from A/B testing because I believe that after 24-48 hours, it's practically irrelevant and results are not immediate. This is a long way of asking you, what do you think?