The version of this trade that makes the most sense to me would be with Hayward and Collins involved.
It's unclear what the Hawks have planned for Collins, but it is apparent they did not view him as the future big man on their roster after trading for Dedmon and Capella. Maybe they view him more as a scoring forward, which, honestly, he might be. But combine those trades with the rumors that the Hawks might have been looking to trade him at the deadline, and I think it may be apparent that the Hawks don't like his fit with Young.
The Hawks can absorb Hayward's full contract into their cap space, generating a sizable trade exemption back to the Cs. This exemption could be used on an expiring contract like Steven Adam or Andre Drummond at the deadline without putting us back into the luxury tax.
The Hawks apparently feel the pressure to put some veterans around Trae Young and try to win. Hayward can run an effective offense while Young is off the court and play off of Young when they play together. This also keeps their cap flexibility for next summer, when they may feel they could be in the running for a big free agent to pair with their dynamic guard. The Hawks also get multiple swings at 3-D players to put next to Trae Young--that don't have a single one yet.
For the Cs, there are four positives. First, it puts off the repeater tax for another year. Second, it gets us a top 6 pick that could turn into a key young piece to put with Tatum and Brown. Third, it gives us a huge trade exemption that can be used to absorb a huge expiring contract veteran for our playoff push without going into the luxury tax. Fourth, it gives us a look at John Collins.
I think without John Collins in this trade, there are still enough positives for the Cs to consider it. But how you view him might really affect whether this trade gets done. My personal opinion is that if CBS can turn Sullinger, Olynyk, Zeller, Amir Johnson (at the end of his career) and even Daniel Theis (who I think had more natural defensive ability than the aforementioned players) into defensive advanced stat darlings, then he could probably do it with Collins. Collins is really, really long and tall. He also has a ridiculous vertical. There are times when he gets vertical, or gets a trailing block on a ball-handler, or uses his quick hands to poke the ball away, and I think, "Maybe he could be a switchy center that is effective in small stints."
Obviously, the greatest benefit is that Collins is one of the best shot finishers in the NBA--at the rim, from three, driving, postups, rolling, offensive rebounds, etc. This dude gets buckets. I wonder if pairing a player like him with Tatum's newfound playmaking elevate Tatum's passing even more. I wonder if having a highly effective scorer inside would pull attention for Brown, Walker, and Tatum and make them more effective scorers.
At worst, he's a more versatile version of Montrezl Harrell, who gets buckets but can play defense. That's a player that can be very valuable to our team.