Just because Crosby and Kessel are penciled to play together, that doesn't mean Malkin and Hornqvist will be married together if for whatever reason Line 2 isn't working. Remember, JR wants four lines of scoring and has done everything towards that goal this off-season. And I highly doubt Hornqvist would have an issue playing on the third line if it came to that. He just doesn't seem like the type.
Personally, I think he'd be perfect in that role: third line five on five, first PP unit. Not unlike Tomas Holmstrom, in fact.
I would LOVE to see Sundqvist force the club to keep him and become the No. 3 center next to Hornqvist. Horny is exactly the type of player you want Sundqvist learning from. And with Fehr out for some time, and with Bonino capable of playing LW, I suppose it is possible that this happens.
I don't think it'd be disrespectful to use Hornqvist on the third line with a rookie countryman instead of either Crosby or Malkin. It's about finding the right mix, as well as the right balance. Four scoring lines (at least to me) means someone needs to play a bit below their talent level to make it work. So a top sixer like Horny plays third line, and a third liner or two (Cullen? Dupuis?) play on Line 4.
My ideal lines would be something like this:
Perron-Crosby-Kessel
Bennett-Malkin-Plotnikov (Johnston would undoubtedly invert the wingers)
Bonino-Sundqvist-Hornqvist
Cullen-Fehr-Dupuis
This would require trading Kunitz, which I am still all in favor of.
OR
Bonino-Crosby-Kessel
Perron-Malkin-Plotnikov
Bennett-Sundqvist-Hornqvist
In either scenario, it doesn't change the power play's top unit imo:
Kessel-Crosby-Hornqvist
Letang/Pouliot-Malkin