May be do it like cod prestige system: You reach leggy >> you can make yourself newbie again >> you get this badge that represents you have reached leggy once >> you reach leggy again >> you get this second badge to represent you have reached leggy twice — these badges must look prestigious and must show their significance.
Ever heard of the saying ;
Human wants are insatiable? I guess it's exactly what's gonna happen. Eventually someone who is able to achieve legendary thrice will still get bored out. The forum works in an interesting way and ranks sometimes just say how much you've been around here.
Users here are not valued based on rank but more of reputation and contributions to the forum generally. As a result of this there are legendary members that based on quality may not warrant the rank and similarly there are member ranks with more organic contributions on the forum than some legendary.
Then again like I mentioned earlier, sometimes rank may just measure how long you've been around.