The change detection threshold is the threshold that defines when two paragraphs are considered the same, but changed, or different ones where the first one was deleted and the latter added. T180259 shows different scenarios where the current threshold may not always bring the best results.
Investigate which if another threshold values bring better results. And document the process e.g. on the test wiki or any other persistent page that can be altered at a later stage and linked.
T181404: Make change detection threshold configurable from php, T182571: Create a test set for wikidiff2 and T183352: Investigate if character runs are better than character counts for threshold are prerequisites for this task.