Thanks you all for your thoughts.
I'd like to show you what I'm building here, and explain a little bit more my vision, regarding the theme page editor.
What I did is I split the big template (like product.tpl) into many small blocks. Then I created a layout mechanism that allows to re-arrange these blocks as you like. There will be additional structures you can use, like section, tabs, columns etc where you can drop these blocks. Of course, dynamic blocks are supported as well (you can place any hook anywhere on the page)
Here's a video. Note that this is work in progress, and very far from the final stage. So excuse the crude design 🙂
Is this something that your guys want / need, or is it completely useless? Any ideas?