I'll probably change my tune if it does end up happening, but I've firmly gotten to the point of just not wanting to bring back Guentzel unless it's for a super team friendly deal (like $8 million AAV at most). I do not like how Sullivan was using him and I really do not want to bring him back just to use him exactly like he was using him before.
I'd be a lot more inclined to bring him back if they would use him with Malkin, but everyone knows they wouldn't do that. I would never shit talk his ability and I think he has shown this year that he's capable of driving play, but this team would just default into putting him back on Crosby's LW and a net front guy on the PP and nothing would change from how it has gone the past few years. Malkin needs a guy of that caliber far more than Crosby does, and bringing back Guentzel would just guarantee that Malkin doesn't get that guy.
Beyond Reinhart, who I think probably stays in Florida anyway, there really isn't much in the FA class that is particularly enticing. Marchessault is interesting, but the Smith experiment makes me far less interested in going after ex-Vegas players. There's also Stamkos but assuming Stamkos will leave Tampa Bay just feels wrong. I think Dubas will likely have to look at the trade route to get another top-6 winger, especially with needing to clear out either Smith or Rakell (or both) to be able to add that.
The other option you have is to stick with Bunting-Malkin-Rakell as the 2nd line and ship out Smith, with trying to acquire a LW to push O'Connor to L3 or just to carry L3 in general. I actually don't mind the idea of targeting Zucker with that kind of move and running with:
O'Connor-Crosby-Rust
Bunting-Malkin-Rakell
Zucker-Eller-Puustinen