I think the Mac game publishers (Feral, Aspyr) would probably prevent you from setting a game to the full 5K resolution, if it was a complex 3D game. Aspyr made a 4K version of Civ V for the Mac Pro, maybe that would be updated for the iMac, but in general I doubt they would shoot for 5K support on many games. 5K is mainly for the desktop.
I am very interested in this product. Having used a Macbook Pro retina for the last few years though, I have grown to like the 16:10 display, and wish the iMac would have a squarer display. But despite that, a Retina iMac would be irresistible anyway. Also wondering if the same event might unveil an iPad Plus.
You jest with the name but I think an iPhone mini really could happen. Apple will be gathering stats right now, watching who is and who is not buying the new models. I wonder if they might see a drop off in sales to female customers and that might lead to the introduction of a mini.