Markkanen is the oft-mentioned name (including in this thread) for them. I wonder a bit about whether (in terms of ball dominance and defense/rebounding) he truly is perfect fit. As Scal noted last night, Porzingis would potentially have fit - though he's fairly duplicative of Chet. The challenge is they need to add rebounding and bulk while still ideally preserving at least ability to sometimes go 5-out. And they can't have Al Horford! So there's likely not a perfect fit, but there's some interesting ones who I think help, even if not necessarily 30+ minute a game guys on that roster.
In terms of profiles I'd think about as OKC, beyond Markkenen.....Wendell Carter is interesting, as is Onyeka Okongwu. Brandon Clarke is that profile, though maybe not quite big enough. He probably isn't available even for a draft pick overplay, but wouldn't you think about JJJ? While he's not as bulky as you'd like, he can space floor and isn't hopeless against size.
Less likely, and also likely not worth the chemistry risk, Julius Randle and to a lesser degree Pascal Siakam are a little interesting (don't see either, to be clear, and Siakam only partially addresses the need for rebounding). An out-of-the-box option is KAT - and no, I don't think that'll ever happen.
Then there's guys who are more clearly role players, but would improve what they have now as a bench 5 - Gafford/Steven Adams types who give size but at cost of floor spacing. I'm not sure they would bother relative to just playing big Jaylin Williams more, but maybe. To me, Claxton and similar bigs don't really help - at least, relative to cost as they won't play that profile 25-30 minutes a game, imo.
I get those guys will cost things - but OKC can afford to overpay for what I agree is a specific fit. And that is why I'm curious how they play it...