Hello,
Thanks for the suggestion, this feature is already on our feature request list. The reason we haven't implemented it yet is that it may be difficult to decide (and probably to understand) what 'next' means. Since you can get to a test from multiple locations (from the todos, the run overview or the tests for a case, for example), 'next' could mean different things depending on where you are coming from.
If you are coming from the todo page, 'next' would ideally point to the next test in your todo list. If you are coming from the test run overview, people would probably expect that 'next' would point to the next test of the run etc. It's probably difficult to get this feature right, so that 'next' always points to the test the user wants to process next.
However, we will look into this feature after releasing the 1.0.
Regards,
Tobias