Not sure how I missed this ...
I, like you, love the Legion. One of those rare DC titles that held my heart for many years, but I honestly believe that they, like many a character or team that can't hold their own title, fail not because of who they are, but because of who they battle.
Quick, name me 5 good villains of the Legion that are not primary villains of other heroes or related to them in some fashion.
Kinda hard. I mean yes, you have foes like Darkseid, Vandal Savage, Mordru and the like ... the "eternity bad guys" and then you have the blood relative or race related bad guys like Lightning Lord, Chameleon Chief, Cosmic King. And of course you have the great Fatal Five (who only work for me when they are a team), but other than that, the pickings are slim. At least as far as I can remember.
Same with most characters that don't have a good villain base. Characters like Moon Knight or teams like the Defenders just have lame ass bad guys. I think the Legion falls into that area as well.
Of course, you could almost claim Darkseid as
the Legion villain, because he was an all forgotten character until they brought him back into the DC realm with that bestest of the bestest "Great Darkness Saga" which we (and everyone else
) both love.