That's not how I was originally thinking about it, but it is an interesting possibility.
For example, I could allow someone to do "+rarity:ultra-rare:season1, -rarity:epic:season2", to get all cards that are UR in S1 and are not epic in S2.
It'll take some extensive editing of the current codebase to allow for this third-level season specifier, but I'll add it to my TODO.