2nd / 3rd line guys? They are 2 of the top 6 forwards in the league. The only guys I would think about taking over them right now would be Kreider, Moses, Nieto. The problem is they get NO production from the 3rd and 4th lines, NONE. The 2nd line has chipped in here and there but in general haven't done much either. I think if you move Mangene between Flynn and Abbott there is still plenty of offense on the 1st line. Drop Diamond to 2nd line with Beattie and Anthoine and you may have something there. Go with an all freshman line as a 3rd line to see if you can develop some chemistry at the very least for next year.
As far as defense and goaltending, I don't see much hope. Sullivan is OK but will almost never steal a game (something this team needs). Marty looks to be a lost cause at this point. The defense is just terrible. I think Heagarty has probably been the most consistent this year, which is saying a lot after how bad he was last year. O'Neil is clearly struggling, he needs a real stay at home defensive, defenseman to be effective (something this team does not have). Nemec, Pryor, Cornell - I could do without all of them. I think O'Connor has some potential to be a player. Rutt, from the couple games I have seen him this year is too slow to contribute this year. Definitely needs to work on his skating/ speeed in the off season.
Then of course, there is the coaching. These guys have tuned Whitehead out for the last couple of years. I don't want to beat a dead horse, but as seen throughout the league- a new voice can do great things even on mediocre teams.