I think it would be great for the users if you add a GUI option in Builder to Create/Edit/Re-order/Delete custom Content Types, Taxonomies and Custom Fields. It would be very useful to be able to create the structure of the website content while defining the front-end.
I believe this will save a lot of wasted time and nerves of all Upfront users in dealing with plugins and it will be greatly appreciated.
I hope you take this into consideration.
To all members reading this: if you like the proposition, please support it so it attracts the attention of the developers.