Good lord no.
The Millsap / Horford experiment already failed in Atlanta, why on earth would you try to duplicate it here?
Our team is way too short as it is, the last thing we want is to spend $30M to bring in a 6'7" PF - that's just ludicrous. We're already getting slaughtered on the boards, and while Millsap is a better rebuild then Horford he is still not a stellar one. Millsap is a nice player no doubt, but he doesn't fit our needs.
We need a proper big with legit size who can dominate the boards, dominate the paint, and defend skilled 7 footers.
Dwight Howard's available, he's already complaining about his role in Atlanta. Other than that, Alex Len is an RFA and the Wiz would probably give Ian Mahinmi away to get out from under his contract. That's the problem - the market on huge space-eaters is really thin and none are great fits in Boston's system. You're better off trying to take the foundation of Atlanta's most recent run and making them supporting-cast guys for (hopefully) Isaiah and another all-star than you are trying to be the second coming of Grit N Grind.
There is more than just free agency in the world. We won't even have max cap space, so free agency is a moot point for us anyway. Only way we can likely acquire a max contact guys is via a sign and trade or by renouncing half our team.
Regardless. we need a guy who can actually help us without our weaknesses. Those three biggest weaknesses are:
1. The ability to defend skilled bigs (Horford cannot)
2. Rebounding (not a single guy on our team averages over 7 RPG)
3. An additional go-to scorer to take pressure off Isaiah
Millsap is an upgrade on what we have at PF, but he doesn't address any of those three issues...and at 6'7" Millsap really doesn't give us any more size then Crowder does at the four, and he's not enough of an upgrade at that position to justify spending $30 million freakin' dollars.
If we want to take a chance and trade for Melo, then fine. Melo is 6'8" / 240 pounds. He can play the PF spot for us and give us an immediate upgrade there, while also addressing one of our biggest needs (an additional go-to scorer). I can live with that move.
If we find a way to bring Howard here, I can live with that. He is a big body who can shut down skilled big men, and he's one of the most dominated rebounders in NBA history, and he can have the occasional dominate occasionally on the offensive end too - I can live with that.
I can live with us making a play for Greg Monroe - he's an outstanding post scorer and a top notch rebounder who would draw constant double teams, leading to open shooters on the perimeter - I can live with that.
If we sign Hayward - I'm not a huge fan of his, but he brings a lot of offensive firepower, he's still young, and he can contribute in a number of ways - I can live with that move.
If we went after a defensive/rebounding bit like Nerlens Noel, then I can live with that - young, lots of upside, athletic, good rim protector, great defensive potential, good rebounder. I can accept that.
But $35M for a 32 year old, 6'7" PF whos averaging 18.1 / 7.7 on 44% / 31% shooting is a terrible move. That contact (especially when combined with Horford's) would completely cripple this team's cap flexibility for the next 2-3 years and he's not worthy of that money...and that's even before we factor in his age and the fact that he could very easily show a significant decline in the next season or two.
Blake Griffin is a very bad idea as well - as much as I appreciate his skill, his health makes him an immediate red flag to me. The Clippers have had some very good teams the past few years, and they have gone nowhere because Griffin is constantly having season ending injuries come playoff time. Doesn't matter how good you are if you cannot play at the most important time of the year, and no way I'm offering Griffin anything close to what he'd be asking for.
I'd make no moves at all (and just put faith in Yabusele and Zizic) before I would spend $30M - $35M on Paul Millsap. That's just a complete no-go move.