I think this is very much a realistic possibility. I think Guentzel and Kessel when he's on are the only guys you can rely on for being consistent goal scorers/snipers. I actually think the Penguins really miss Sheary because of that, they really need to get a guy who can put the puck in the net for the bottom-6. Kessel is supposed to be that, but Kessel has been very hit and miss at ES this year IMO.
Sheahan has a good shot, but he's not a sniper. I'm tempted to include Pearson in that group too. ZAR and Hornqvist are grinders. Rust hasn't looked good this year when not with Crosby. Simon has a muffin of a shot. I don't think it's just as simple as "get someone with a cannon of a shot", I think there are a lot of issues that need to be solved with that 3rd line. I think the biggest issue is that the 3rd line has never had the speed it needs to be effective. If you had a speedster RW that could grind and retrieve pucks for you, a Pearson-Brassard-RW line should work well, the Penguins just don't have one of those.