Customizing WordPress is one of the rare things the typical WordPress developers avoid to attempt to. I have been a core PHP developer since last 3+ years and I have been working on developing a friendly DIY website builder customized from WordPress, at http://godigie.com. I am a newbie on Freelancer.com, but considering that our goals are similar towards the betterment of website development, I am deeply inspired to work on this project.
I have made the following customization in WordPress till now.
1. URL modification - Hard-coding
2. Multi-site Network establishment and related modifications.
3. Default Plugin Installation.
4. Theme Redesigning - Hard-code, Bootstrap
5. Plugins' enhancement - WooCommerce, Contact Form - 7.
6. Theme Showcasing - Galllery Plugin Modifications
7. CMS Makeover - Dashboard UI modifications - Hard-coding and Plugin enhancement
Also I have created 75-80% SEO and Webpage load time up to 2.5 seconds by caching plugins and hard-code.