I think they want the other 20% cdr item to be something that is bought instead of Morellonomicon, not in tandem with it.
Lich Bane is very attractive on some assassins, and so is Morellonomicon for resets and kill securing, giving them a quick one-two punch to 40% cdr and tons of killing power would be a crazy power spike and pretty much remove any thought from their build paths.
Mages that DON'T build Morellonomicon either do it because they don't need mana (efficient early costs or manaless) or they prefer a different mana item (stacking Tear or RoA) so the other AP/CDR powerhouse should probably not benefit mana in any way.
I think Tear stackers represent the champions most in need of a big cdr option. I like playing Cassiopeia and she is all about lategame dps that would benefit a ton from capping her cdr, but after she builds her core items (Tear, Usually a Rylai's or RoA, Rabadeedlies, and Void Staff) there's very little room for enough cdr to cap out and maximize the number of Twin Fangs she can output even though she uniquely doesn't need to buy boots.
I think Rylai's would be a very good candidate as an AP/CDR option. It's manaless, completely different from Morellonomicon in the niche it provides (kiting instead of kill securing and resets), is a great item on many champions who don't usually fit Morello into their build (Vlad, Cass, Karthus, Kennen...) and could be altered to build out of a kindlegem because that more or less makes sense. It also synergizes with the passive by allowing champions to continuously reapply the slow, and makes the Rylandry's combo more viable by adding cdr to a combo that seems to be about adding decent damage to utility champs who would otherwise be spending two full item slots on no cdr which should be just about their most important stat.
The only issue I think is that its passive is very powerful and priced appropriately so it might be hard to make room in the item's gold budget for the cdr in addition to being a powerful health and AP item.
from 
. Bother give you a burst of AP damage, bother give a lot of AP, and both give movement speed. Lich Bane could be the CDR choice when deciding between the two.
, the tank sheen item, and 
the AD sheen item both have 20%. The AP version, 
he just got nerfed, so you want to bring him back don't you.
.