It's good that you want to improve your skills, but this is not the way to do it.
You should work on an open source project.
There are plenty of large projects that would be thrilled to have a developer who wants to tackle something more than a small bug, but you have to go find them.
Moreover, open source projects are much more deserving of your talent than the people posting on this thread trying to get some free work out of you. Don't undervalue yourself.
What are you interested in?
Personally, I have contributed to Python, Django and AngularJS because I use all three on a daily basis. Think of something that interests you and I'd be happy to help you find a project that fits your goals.
You should work on an open source project.
There are plenty of large projects that would be thrilled to have a developer who wants to tackle something more than a small bug, but you have to go find them.
Moreover, open source projects are much more deserving of your talent than the people posting on this thread trying to get some free work out of you. Don't undervalue yourself.
What are you interested in?
Personally, I have contributed to Python, Django and AngularJS because I use all three on a daily basis. Think of something that interests you and I'd be happy to help you find a project that fits your goals.