In theory, you now have the fundamental knowledge you need to build any digital device. You can take the basic gates discussed in this article and construct anything. However, it is often more convenient to use larger-scale devices so that you don't have to combine 50 chips to build something common like an ALU. It is also helpful to see examples of different ways to combine gates to create complicated systems.
If you would like to work on a bigger project, you can try building the digital clock described in How Digital Clocks Work. If you want to learn more about TTL devices, the following books will be helpful:
You will be AMAZED at what you can create with just a few ICs and some creativity. Have fun with it!
For more information on electronic gates and related topics, check out the links below.