A long time ago I built "squeeze page" manager which had an editor like this (it was simpler and more focused on content management). I sunk a lot of time into it. However, I ran into the same problem I bet you guys will run into: "the design tool chasm". On one end are simple tools for publishing on the web (Weebly and Wordpress) and on the other are professional tools (Photoshop and Sublime Text 2/Textmate). In between are tools that no-one really wants.
Basically, don't stop at the editor. Its a feature. Take a look at Unbounce or HubSpot for "marketing tool" inspiration and Weebly and Wordpress for "publishing tool" inspiration. Or, if you want to be used by professional designers, you'll need to go in a whole different direction (which is likely the one you are on now). Of course, if you have target customers that love this as is, their feedback vastly trumps mine. But never stop talking to your target customers (steal them from the big boys with your personal touch and solve their problems)! Good luck guys!