I've been playing Sylveon GX/Gyarados BUS (on ptcgo, in standard). It's quite fun. I used to play Gyarados SF in the day---so it's a huge throwback to be discarding karps again. Sylveon GX is a big consistency boost--protection from being donked, moderate damage output throughout the game, easy access to resources early to discard/recycle karps or set-up Gyarados, and a way to apply pressure before/between chaining Gyarados (since they usually take two turns to set up). Gyarados provides much higher damage output when necessary and only gives up one prize, which helps a lot.