Yeah, I think Boogie becomes the main (and actually realistic) target this year at the deadline. Unless either A) Brooklyn miraculously has a competitive start to the season, or B) Sacramento actually figures a few things out and is in good position to be in the playoffs this year, which are both pretty unlikely, then I think there's actually a pretty good chance that Cousins is a Celtic by February. There just seems like too much happening in the right direction for that trade not to happen at this point, especially if the Brooklyn pick is looking as promising as it is expected to be.
If we don't get Boogie, then I agree with the Hayward talk as a realistic target next year. Could he play the small-ball 4 full-time next to Horford, though? If he could, then he'd actually be a pretty good fit for us, but hopefully KO will finally take the next step and prove that he's a good longer term solution at the 4 for us.
I also wouldn't mind someone like Ibaka, who I think would be a near perfect fit for our system at the 4. A lineup of IT, Bradley, Crowder, Ibaka, and Horford is title-contending in my opinion, though I wouldn't max him out by any means.