First of all it depends on the scope of the project.
If you want to build a CMS to suit your specific niche needs, then it is a good idea.
But if you want to launch a CMS as a product to compete other CMS in the market then you need a feasibility / reality check.
So first clarify this in your question.
To launch your own CMS, you need to describe your Website / Business needs, so that I can suggest you better.