I understand that you said you don't want to change the list, but there are a few items that have niches that are unusable for literally no reason. I don't see why you wouldn't just switch a few items on the list to give a couple more options.
You should remove Multitype, Stance Change, Zen Mode, Flower Gift, Forecast, Friend Guard, Symbiosis, Telepathy, Healer, Plus, Minus, Illuminate, Honey Gather, Run Away, Teravolt, and Turboblaze from the list of item-abilities since they either don't work, do literally nothing in singles, or are redundant.
This would give you room to add things like Flame Orb, Weakness Policy, Air Balloon, Damp/Heat/Smooth/Icy Rock, Cutsap Berry, Salac Berry, Expert Belt, Red Card, and maybe a few damage reducing berries, which all have small niches but are currently unusable.
The only downside to this is a few posts here might have outdated sets if they use any items that you add back in.