The idea that a character like Rex or another 1st party character getting in being "old news" isn't really the point. People like the character. Japanese fans sincerely love XC2 and Rex was a frontrunner for DLC.
XC2 also has Torna: The Golden Country. Sakurai has played, and loved the game, and it happened to be during Smash Ultimate's development time, so I imagine that had something to do with the Mii Fighter costume.
I simply don't think, after every fan rule has been BTFO, game after game, that we should start making new fan rules for future DLC characters. I would simply lay out the "eligible" characters as anyone who isn't already a playable character, a boss, or Assist Trophy. Maybe even lump stage elements in there.
The eligible characters, potentially, are anyone who happens to be a Spirit or a Mii Fighter Costume. That's my opinion anyway.