I've been down this road before (two custom CMS platforms built over the years), and to play devil's advocate here: do you really need to build a custom CMS?
What problem are you up against that aren't being solved by existing solutions? If those problems aren't fundamental, how much time and money will be saved by creating plugins for an existing CMS?
I know the temptation to start from scratch, but if this is something to be used in production, the todo list is staggering: you'll need user authentication, security on the front and back end, admin interfaces, plugin infrastructure for extensibility, theming (if this will be used for multiple clients), third party integrations — and that's all before you get into really annoying and notoriously finicky shit like media uploads and i18n.
If it's for learning or a hobby, go nuts, but if you're planning to use this with clients, save yourself the heartache and late nights and use something established and open source. Join the community and help fix existing solutions' shortcomings — that's a bigger overall contribution to the world.
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.
Why reinvent the wheel? It already exists in thousands of forms, many of which are open source.
WordPress, for example, was a fork of another abandoned open source platform and it now powers over 26% of the web.
But if you must undertake this odd adventure, make sure it's different with a purpose and solves problems others aren't.
I have built my own CMS called Butane for my web design clients which was an easier version of WordPress.
I know the other people were saying not to reinvent the wheel, but if you are really interested in building your CMS, it is not hard if you know PHP and MySQL.
First of all, you need:
1. A MySQL or similar database
2. A scripting language like PHP or .NET
3. Clear objectives and goals on who you will make it easier than the popular CMS software.
Also, what I did was create these for Butane:
1. a menu system
2. an back end administrative control panel
3. a user manager
4. responsive layout that can work with any HTML and CSS.
Basically, your PHP or other code will interact with the database through your URL and if you use PHP like I did, you will also need to modidy the rewrite rules in your .htaccess file.
1. social media URLs stored in your database and manageable through an HTML form
2. SEO options like meta tags
3. ecommerce. I just use a Paypal link which most of my clients had
If you have any questions, I can help you with this.