That has never been the case.
Ubuntu is always derived from the testing branch.
With the occasional overlap with stable, such as this current development cycle.
https://launchpad.net/ubuntu/saucy
For the OP, forking and modifying are going to be a lot harder to accomplish than you'd really want to do.
Distributions like Debian and/or Ubuntu and even Mint have hundreds to thousands of active developers working on projects full time. In some cases, like those working on Debian, they might work on the project over several years.
However, I don't want to be a total debbie downer on the issue, so there is nothing wrong with building your own highly personalized Ubuntu spin, for your own use only (as well as for friends/family)
If you can follow this,
https://help.ubuntu.com/community/LiveCDCustomization
it might be helpfu for what you want.
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