I think Lip has a great chance to be playable.
I don't.
Yes, Lip has had her series represented since Melee. Yes, Panel De Pon was a bit popular at a time. The question I worry about, though, is "Would today's audience know or care about Lip? Is it worth bringing her back?"
In this installment, Sakurai is pretty clearly focusing on characters that are well known and instantly recognizable by the audience, as well as characters that have been promoted a lot in recent and upcoming games (Rosalina and the Fire Emblem characters are good examples). Lip has too many obstacles in her way to be considered amongst them, I think. She's obscure, she's Japan-only, she's rarely requested even in her home country now days, and worst of all, her series is currently
dead. I don't really see Sakurai going out of his way to bring her on to the roster over so many other characters unless IS has any plans to revive Panel De Pon.
Sakurai has added obscure characters as retro reps before, but I'm not sure if Lip even has the same qualifications that previous retro characters have had. Her playstyle could be unique but I don't think it'd be groundbreaking (Ice Climbers), she doesn't have extreme seniority or importance (G&W, ROB), and as far as I can tell Nintendo isn't planning to do anything extra with her (Pit, sort of ROB). She just feels really unlikely when it all adds up.
Please prove me wrong, Sakurai...I want her so badly.