Marner is one of the only players that is around Petterson's level that is theoretically available. His rights would probably be traded for things that we don't want to give up. And would cost 13-14 in cap. He's also 27 and a right winger.
Rantenen is 28, is also a right winger and would cost 13-14+.
Are these the situations you are dying to get Pettersons 11.6 off the books for ? At least they are high ceiling moves and are still better than a glorified Miller trade.
But I think the point stands. Petterson is already here , is a centre , is 11.6 and not 13+ and just turned 26. That 11.6 is best to stay right where it is.