As much as I'd love to see him return- heck my username is a pun off his name- I don't think so. Background character at best, but it'd be a pleasant surprise to see him in. But I won't hold my breath.
"When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up." - CS Lewis
I think Petey is more then a background character. He represents the Piranha Plant race, much like Bowser is the "Mega Koopa" Petey is the "Mega Piranha Plant" just holding no authority over them (as far as we have seen).
In other words, Petey gives the player a option to play as some from of a Piranha Plant, a option that was never there before. Therefore, to eliminate Petey seems rather counterintuitive to me.
Also, Toadsworth was never in any multi-player games to begin with so I don't think his case is comparable.
P.S. Sorry I misspelled your name in my previous post Megaleg.