I've seen some interest in speedrunning individual missions and/or sidequests. Could have it Start a capture when a new quest is grabbed, and have it stop when a Quest is finished, then have it look at the amount of time it took, compare it against other saved times in a database (on web?), and if it's in the top 20% save the replay (if not then it ignores/deletes).

Could have it clip the best moments in a PVP battle in Random Arena (or Guild Battles, Alliance Battles Etc.) -could use DPS, HPS, Clicks per second, or Number of spells used in a 5-10 second period as "Heat/Intensity" detection.

Clip most intense segments of Boss Fights (using same criteria as the PVP battles mentioned).