granting an ability mean being possibly vulnerable to gastro acid etc. And having one more ability have skill swap repercussions etc.
So...
Casteliacone Deluxe: Boosts the BAP of Rollout by 3 and causes its BAP to also be boosted by non-consecutive usage. Increases Defense and Special Defense by 1 rank. Causes the Pokemon to always move last within its priority bracket, regardless of Speed. The Pokemon's attacks will never miss an opponent and opponents' attacks will never miss the Pokemon. OHKO moves have normal accuracy when used with or against a Pokemon with this item.
Affects: Lickitung, Lickilicky.
Dueling Gloves: Boosts the BAP of Double Hit by 2 per hit. Increases STAB bonus by 1. Boosts Atk rank by 1. Fling causes the target to become taunted for 3 actions. The holder is not considered encumbered.
Affects: Aipom, Ambipom.
WRT Relic Crown, thoughts? I did see Glacier's proposal and I have serious problems to how it boosts too much a typing that Yanmega pretty much doesn't use, Tangrowth uses sparingly and Mamoswine uses quite a lot. Mamoswine has screens, stone edge and some other rock moves. Tangrowth has rock slide, ancient power and reflect (so like half rock utility) and Yanmega has Ancient Power. And that is all. Finally, being good vs sand is useless for tangrowth and yanmega for the most part, since tangrowth will mince rock mons regardless and yanmega will be minced with or without the spd boost.
So yeah, I am heavily against focusing on giving STAB, as it is a very uneven option.
Also, explaining for things: the item gives two 9bap rock moves, one special and one physical. Mamoswine can properly use the physical and Yanmega the special. Tangrowth can use both, so it has a small advantage. That is why I put +1 stab, so Tangrowth gets +1 one type and Yanmega/Mamoswine on two types, possibly evening the difference.
Just to explain.