7.0 Beta 1
Pre-release
A lot of big changes went into this release, so a beta seemed like a good idea. There are also a lot of changes that aren't backwards compatible so some people may opt to stay with v6 if they are using rename in scripts or anything.
The biggest change is that instead of our own variable parser and replacement engine, we have switched to the html-templating engine, Nunjucks, because it offers some insanely powerful features. Read the 7.0.0 branch and Nunjucks documentation for more info, but the biggest advantage is that you can write filters to modify variable values and chain multiple filters together to accomplish things you could never dream of before.
Another change is how file indexing is handled. Before a variable had a property to tell the tool whether the value was possibly unique. That's scrapped and now any time there are conflicting new file names they'll automatically have an index appended.
Read the docs for more info.