If you were tasked to think of one idea to make Eevee completely overpowered and broken without evolving, what would you come up with?
…well, without thinking too hard about why we’re doing this, I’d probably give her an ability that sort of combines Conversion 2 with Protean – Eevee, the ultimate adapter, automatically shifts her type to gain resistance or (if possible) immunity to all incoming attacks. That’s immunity to 8/18 types and resistance to everything else. Slap an Eviolite on that and it’ll survive damn near anything, up to and including Primal Kyogre’s Origin Pulse. Of course, I don’t exactly know what you’d do with Eevee at that point other than maybe Baton Pass some Curses, but you said “one idea” so that’ll have to do.