I was almost 100% sure I was going to upgrade my old late 2010 13'' MBA, and my candidate was the 12-inch MacBook when they added support to 16GB RAM (which just happened). However... I'm now in front of my 13'' MBA, I had the other day a 12'' MB, side by side, and now I realize I really want 13'' at the very least, and hopefully more.
I really believe Apple could create a very light 15'' MacBook (non-pro), with a weight very similar to the MBA, perhaps only slightly heavier or even the same weight.
I'm in a weird situation, where my current MBA has the specs I really desire (MagSafe, at least 13 inch display, NVIDIA GPU -yes, the old 320M, but it lets me compile CUDA code), while the current MacBook offerings would give me more performance and more disk space, but I'd lost too many things I currently enjoy).
I think I'm going to extend the life of my MBA a bit more. If Apple releases a light 14'' or 15'' MacBook at some point, then I think it will convince me to buy it, but today I feel like buying a new one would be money I won't enjoy as much as my current MBA.