@DisplayedName Doing this outside the game would just be too much hassle for most people. It reminds me of marketplace websites for games like Warframe, and how much I absolutely hated using them.
However, back before the current RDF system was implemented in WotLK, there was a simpler LFG system that functioned a lot like what you just described. You basically select the content you want (dungeons, raids, even PvP, quests and openworld bosses) and enlist your character, writing a short description like your role, ilvl and spec or whatever, and it would list you for all people that are using the tool to search for people for said content, and they can whisper you and invite you - there was no automation, like RDF has currently that automatically creates groups and teleports all of you to the dungeon - and I think that's important, because people have forgot how to communicate in such dungeon groups thanks to such "conveniences", helped further by the drop of difficulty of all dungeons, so they can be completed by brainless button-mashing monkeys, and so any communications about pulls, CC and boss mechanics are completely unnecessary (lol, remember when we'd have to mark moon on a mob and tell the mage to poly the moon? Pepperidge farm remembers!)
But having it ingame was actually very convenient, because you could list and unlist whenever you wanted, and the list would update in real time, and you wouldn't have to minimize the game to open your browser or whatever.
If they could bring back this system from vanilla and TBC, I think it would do some good, but I fear the convenience from RDF is too embedded in people's expectations of modern-day MMOs and there would be a huge backlash if it were to be removed.