I planned a million different barns before we bought our property and the barn that I am building is none of those. The way of your land and the amount of money you end up having changes everything. Still, it is a lot of fun to plan barns.
First think of all the barns you have been to what did you like about them and what did you not. Try to think about what did not work out in those barns or what about them made something hard to do and then try to think of a way to do it differently. Google barn plans and barn builders they tend to have floor plans available to look at.
The one thing that I did not think about at first was hay storage. I like to have a few months worth of hay for my horses, especially in winter when hay prices go up and the same hay that you bought for $4 a bale is now $5 or $6 a bale.
There is a barn builder in Georgia that has a lot of really good plans on his website is Bentley Horse Barns
(warning there is annoying music when you open the site sometimes) I think he even has plans with living quarters.