I don’t think WotC has that copyrighted so I think it should be fine for fair use 
The trait would be Tier 2 and would be “Preferred Enemy XXXXX” you would insert a card type “wildfolk” “dragon” “human” “knight” etc you get the idea.
It is like the slayer trait but instead of skull damage it would be spell damage and it would deal 50% more spell damage to those types.
Um… yeah for completeness I suppose it could be done… but sorry not getting very excited…
Generally these types of things are just baked into the spell themselves, so traits like this would be costly filler to get the desired effect out of a given spell.
I agree with Mithran, that this sort of effect is best put right in the spell itself. See: keghammer, desdaemona, gob-chomper.
But they are few and far between and mostly revolve around devour and mana color