Let's think thematically for a second. What sort of actions results in spoils? Pillaging and looting, clearly. Looting has been mentioned above, and I think it need not be an attack--possibly even some interaction with ruins or shelter or something. It sounds to me like it would relate to trashing, but from your own hand, not someone else's.
So what other sort of thing can you do, that results in gain you would call spoils? Economic and property crime, like theft. Hm... that would actually be a pretty interesting Thief variant: If you trash a treasure, you gain a Spoils or two. Early game, thin your opponents' decks in return for some limited-use high-impact acceleration. I like it. Mid-late game, similar to real Thief but probably better.
So what else is associated with Spoils... conquest and war, obviously, but that's strongly overlapping with Pillage already. How about, spoils are associated with claiming new territory. Imagine a Hoard-like card, cheaper and maybe gaining 2 at a time. Awkward card, for sure, but doable.