Actually, having Plekanec on the third line would not be that crazy.
Let him take on the full defensive duty with DLR and Semin. First line PGG, Second line EDDKASS with soft matchups. Can Eller thrive in soft matchups ? He'd be the best bet with DD and Kassian, but these two thrive with soft as marshmallow matchups. Is that wasting Eller's potential defensively AND Plekanec's potential offensively ? Hear me out.
Plekanec WILL produce no matter with whom or where you put him. Give the "bottom 6" the defensive duties, AKA Mitchell and Plek who are really strong defensively and give Galchenyuk and Eller the softer matchups. Our only weak players defensively are Galchenyuk, Kassian and DD. DD and Galchenyuk has produced in harder roles, but not as a C. DD as a winger matters less and Kassian absolutely needs to be sheltered defensively. So we have two players who need "shelter". Be that as it may, Kassian and Galchenyuk should be with offensive players to succeed. Plekanec ? Who gives a damn. The guys has been producing alone since Cammy left in 2012. 3 seasons in a row he's been alone and he's been producing 50 points.