I think if Ariel Eberhart came back it would be AMAZING because she was such a fun, unpredictable (funpredictable, forgive me) character to have around! I can just imagine her being cruel and mocking Nick over the loss of Juliette–which is terrible but it would fit with their past interactions and the history between them.
But I also agree that now that Danielle Panabaker is busy with The Flash full time, it would probably be difficult to fit filming an episode or episodes of another series into her schedule. That doesn’t keep my fangirl heart from wishing for a reappearance at some point. She’s high up on a list of seemingly one-off characters I would love to see return (I died of happiness when they brought back Lt. Orson).