I would think Siakam, plus some sort of combination of Powell, OG, Boucher, Davis and probably two 1sts.
Siakam was All-NBA Second Team last year, is only 26 and on a contract that doesn't expire until 2024 – so I think he's actually a really great asset to have.
Maybe Siakam, OG, Davis and a 1st round pick? But it seems like a runaway for Houston if they were able to land all of that for a disgruntled Harden. I'm not sure.