A clan as a dying entity.

I'm certainly no expert in Heroes, but I could suggest that Clans might be useful for the following reasons:
1) More experienced players in a Clan can give optimal advice to beginners, being less stingy with secrets than, say, on a forum.
2) In a Clan, you can set up pairs or small groups for team play. For example, if the map and Factions are known in advance, the players can roughly plan who will go where, why, who they will kill, and how they will build their base before the tournament or whatever you have.

And there are all sorts of minor things, like creating a special atmosphere of communication within the Clan, which in itself can attract some individuals. A ranking system for players who like to compete.
A Clan, as a system, is capable of adapting more successfully and quickly to new features in Heroes (translations of texts released abroad, for example, because things usually take time here) + serving as an indicator of people's involvement in the game.

In general, I don't think the idea of Clans is outdated. All that's needed is a leader who is willing to get involved and set goals for the participants, taking into account their desires and abilities.