If I may put my 2 cents in here
As someone who likes to play JG Karthus, I feel like one of the reasons I almost never upgrade to Runic Echoes is because of the fact that it's just not as useful to me as say, just upgrading to Blue Smite (sometimes not even do that) and then work on either Tear or Haunting Guise.
Perhaps the problem is the fact that Runic Echoes itself is just a slightly watered down version of
, an Item that is popular on Burst Champs, but not so much on DPS champs, which correlates to the fact that Junglers who use Runic best are champs like Elise, Nidalee, and other AP Burst champs/assassins.
Maybe the solution lies in making a new JG Enchant? Something that is a good fit for DPS Mages but not as much for Burst? (and something Ezreal can't break preferably lol) I'd consider either putting in Guise (because it's literally only ever used for 1 item), or Catalyst (because a JG Catalyst item would be cool)