It should be pointed out that the Raps, Rockets, and a half dozen other teams will probably be willing to give up more than marginal assets for Millsap.
Milsap could potentially make Toronto pretty dangerous. That'd be an interesting pairing, though it's unclear what they'd trade for him - Ross and the Clips pick this year? Could Ross be their next starting two guard between Schroeder and Bazemore? Not too much raw talent there.
I like the Rockets fit less. He'd certainly help their D, but with how they're playing Anderson is a pretty good fit at 4 for them. I don't think Milsap shoots the long ball well enough to justify that change. They're looking more for an upgrade at the 5, with Koufos in Sacramento supposed to be their main target.
OKC would be another really good fit for him, though it's unclear what they could trade for him, too.
Denver apparently was close to trading for him over the summer and thinking about it, I think he's the perfect kind of player for them to target. Denver has the exact same problem that Boston does: an excess of role player depth with a lack of superstar or even elite talent. The Nuggets and the Celtics should be looking for trades that allow them to consolidate their talent.
I'm not sold on the Millsap/Horford pairing and I wasn't while they were playing together in Atlanta. I don't think reuniting Millsap and Horford fixes the Celtics' existing rebounding issues.
I think the Hawks backed out at the last minute due to thinking they could get more for him. And that's what I'm reading is the general impression of other GMs around the league - the Hawks want a substantial return for Milsap, which puts in doubt whether they can find a deal for him before the deadline. Of course, they could always change their tune and accept less when it comes down to it, too.
I think the more interesting question is this - what happens with Howard if they trade Milsap? They're also listening to offers for Korver and Sefalosha, too, so it sounds like they're looking to go young rather than pay these guys.
But in that scenario, it doesn't really make sense to keep the 30 year old Howard with that type of team. I'm not a big fan of Howard, but he would certainly help our rebounding and rim-protection issues. He's also been a defensive menace for Lebron in the past, and he's on a very reasonable 23.5M deal for only two more years after this season. And though limited offensively, he still gets plenty of "trash" points and would represent and excellent PnR option with IT with him rolling to the basket.
The question would be how much he would cost. I'm not sure that I'd give up one of the Brooklyn picks, certainly the 2017 swap, or Brown for him, which they might expect in any deal.