From a licensing perspective, you cannot legally do that without having another license for that subsite.
From a technical perspective what we've done in the past (simply to update the parent logo area with a sub parent logo and name) is:
- clone your existing master page template and rename it to your subsite masterpage template. You will use this for all subsites.
- update all the css, js and other references for your new style/design
- create a custom page type called
Subsite and add a few fields to it like Name, LogoUrl, etc.
- add a new subsite below the root level of the site
- on the new master page template, add a repeater for the logo section and grab the logo from the Subsite page type property
LogoUrl
- continue on doing the similar thing with the other subsite page type properties.
PRO TIP:
For the paths in your webparts, make sure you use dynamic values. For instance, /{0}/%. If you're on /Subsite-1/Home it will return all pages below /Subsite-1. If you're on /Subsite-3/About/Contact-Us it will return all the pages below /Subsite-3. This will allow you to have one set of subsite templates and use for all subsites without having to create a new set of tempaltes for each subsite.