i was reading today that Holmberg was practicing in the top 6

I like Kampf for his role-- but I don't think he sees regular top 6 action. More likely TOR shifts Nylander, Jarnkrok or Kerfoot to C, or bump Holmberg up.

Marner goes top line with JT & Bunting.

i was reading today that Holmberg was practicing in the top 6

Yeah, I can only put one person in each line and based on the last couple or so games, David Kampf (at least to me) has had more faceoff wins (without looking it up, just going by watching the games, and correct me if I'm wrong), so I figured, while Auston Matthews is out of the line-up, David Kampf can take the second centre line. And eventually, Kampf and Holmberg can always switch lines (as second centreman and third centreman) for the next three or so weeks

You guys should try Nylander as center. Has it even ever been tried before?

Yes it's been tried. Didn't look great-- but it's not been recently-- Willy has grown a lot since though, and could be better.