Well, i’ve take a look at your link SiNNuT.
This already possible with MODx.
In Revolution, you create a template and then use Templates variable to change your layout.
It’s up to the designer to create the containers because MODx doesn’t provide the "block" facility of almost all existing CMS.
But that was my point, it IS the good point of MODx. It does not enforce you to use its facility, you have to build it by yourself.
Since MODx let you choose your markup, it’s not possible to give designers default template behaviour for a common template interface.
We (as designer) can provide guidance that you can anywhere on the web.
But nothing justify the right for MODx to impose the use of "sidebar" instead "aside" or "content" instead of "main".
That’s being said, MODx (Revolution at least) is flexible enough to allow a more fine control "a la concrete5" with the help of a custom manager page.
I’m not saying that it would be easy, but it’s possible.
ExtJS (and little PHP) can easily put up a page like this one:
On top you can imagine the related TVs that are changed when you select a template form the below list.
It may be a good idea to propose something like that to the team with less intrusive behaviour than in concrete5.