Are you thinking of having a library (or even market) of these designs, or is your goal more to allow each site builder to personalise their site in fine detail?

If you are thinking of the latter, or both, maybe you should consider having a couple of layers of customisability. It would be nice to allow simple customisations (colors, logos, even basic layout) without getting into templates. Perhaps with a GUI, but possibly even just something standard like CSS. Even as a technically minded person, I would balk if I had to learn some custom syntax to achieve things like this.

Then underneath you could have a full templating system. If users want full control, they should understand it will come with a learning curve. Even for this, consider starting from a known base (e.g. Django templates), and just building convenient constructs that fit your site (e.g. custom tags for common tasks).