Hi, welcome to BYC!
Some of the things we discovered were important to us in a coop are:
* Make it easy to clean
* Make it tall enough for you to stand up in
* Allow room for a waterer and feeder
* Make it as invasion proof as possible to keep predators out
* Allow for a lot of ventilation
* Make it roomy for the birds, the bigger the better
* Run electricity to your coop for lights, fan, etc.
Also, at some point you may need to quarantine a bird so figure where they are going to go. Quarantine is important when you add new birds or have a sick or injured chicken.
Good luck to you!