The following non-slotted middle headgear would be obtainable from killing certain MVPS (the percentage to drop would be up to you) or by doing certain quest(s).
Robot Ear from Kiel D-01 (Kiel is a robot)
Bloody Angel Feathers drop from Incarnation of Morroc (Angel) (item description says fallen angel w/c perfectly suits the Angel from DG since they are shadow property)
Odin Mask drop from Valkyrie (item name has 'Odin' in it, then the sonic blowing Valkyrie comes into mind)
Hairband of Reginleif drop from Valkyrie Randgris (item description says Valkyrie so I thought of the biggest Valkyrie in the server, Randgris)
Ifrit Eye drop from Ifrit (item literally has 'Ifrit' in it's name)
Ears of Ifrit drop from Ifrit (gee, I wonder if there's an MVP named 'Ifrit')
Magical Booster drop from Vesper (item description mentioned Juperos so I thought of Vesper)
Mask of Aegir drop from Kraken (Kraken's the only MVP that I can think of that is under the ocean)
Angel of Ghost drop from Gloom Under Night (I thought of Evil Snake Lord at first, but he's weaker than Gloom even though they're both Ghost property)
Anemos Mask drop from Mistress (item description says wind, thought of Mistress)
Mask of Gaia drop from RSX-0806 (item description says earth, thought of RSX since he's a miner mining the Earth)
Black Devil's Mask drop from Berzebub (Decided to give this to Bee since Fallen Bishop Hibram already drops )
Ice Ear Wing drop from Ktullanux (because Garm is camped too much)
Peco-peco ears quest-able just like the Peco-peco hairband quest
Imperial Feather quest-able by exchanging like 200 or more Talon of Gryphon
Sigrun's Wing (reward included alongside the 200 bronze coins from doing the Traditional Rebirth Quest, thought of this since the item description says "Ear adornments that symbolize rebirth.")
These are the idea drops that I had so far. I thought of it because it sucks thinking that most of the middle headgear are only obtainable through the chiqita box (RNG) and Reward Guru (w/c) costs 5 tc.
Tears if this suggestion gets declined.