Diversity behind the replays added.
Like I wouldn't mind seeing some of the record book replays getting in there, if only just to show some of the limits that were pushed with this game, and to hopefully inspire some people to break them. A couple years ago it had a spike of activity and competition between players; now it feels like one record gets contested, not necessarily broken, every year at best.
ALSO, LET'S NOT FORGET THE MAJORITY OF PLAYERS PLAY MULTIPLAYER, NOT SINGLEPLAYER. I understand how using the default replays to show off some really good looking sets can be beneficial, so why not just kill two birds with one stone by getting multiplayer replays from the people who were chosen for having good sets? It doesn't need as many replays to showcase more sets, and it also can show some example games of some of the game modes. The lack of tk replays that made it into the default replays always surprised me, considering they can be some of the most impressive mp games.
The default replays are technically not finite in size, in theory you could make them as large as you want, but it does make sense to create a level of prestige to get into them, so there's an imaginary cap on the size of the default replay folder. That being said, not everybody can earn their way to fame through singleplayer, so it feels a bit egocentric to demand that it consists of only good singleplayer replays. A good multiplayer replay, not even necessarily a spar, can still be default replay worthy.