Thinking of the URL structure of your future site is a very important step. Give it more time to make sure you plan everything ahead.
One thing to take into consideration here is whether your site URLs need an extension at the end or not (examples: .html, .php, etc).
Two contradicting points of view are based on the following:
No URL extension needed: This will ensure you will have no problem with any possible technology changes (e.g. moving the site to a new content management system in the future).
Yes, do add URL extension: (Voiced by Matt Cutts in this video response) Although there is no direct impact on rankings, an URL extension makes it clear both to a user and a search bot whether this is a page or subdirectory.
What’s your opinion? Do you prefer to add the URL extension? Do you think there’s any difference at all?