A feature I would like to see for the next generation would be to give template designers the ability to create variables used in templates in order to allow template based translations. When developing a custom template it is extremely difficult to maintain all of the default language variables and it would be nice to still give those who need it the ability to translate our templates.
The way I envision it, this should be possible. They may be referenced slightly differently, such as:
{{ trans("mytheme::stats.last_user") }}
Where "mytheme" is the name of the package (unique name referencing the plugin/theme). We haven't done any work on the theme system as of yet, but we are planning it and working out what we need and how it should work.
To further add to this I would suggest theme based languages that override MyBB's languages. For instance, when coding Rebil I had to modify a few core language variables resulting in me having to supply the modified language file with the template.