This is a trade idea focussing on getting young players that fit the Scottie Barnes timeline.
Chris Boucher or any combination of Toronto's expiring contracts (eg. Gary Trent Jr, Thad Young, G. Temple, O. Porter) that fit into the $23 million dollar John Collins trade exception.
DeAndre Hunter (or B. Bogdanovic, but I think Atl would prefer to keep Bojan over DeAndre)
A. Pokusevski (injured. Will likely not be resigned)
The important players (not negotiable) in this deal are in all CAPS. The other players are decent players or for salary matching purposes.
Why would Atlanta do this deal?
They get Siakam + ?Boucher. Trae Young is in his prime and this trade surrounds him with two high caliber talents to make a deep playoff run. If things go well with this experiment, there is a good chance Siakam will likely resign. They get out of the salaries of Capela and Hunter. Trading Capela will open up playing time for Okongwu. Trading Jalen Johnson is certainly a high price (Toronto would also love to get Bufkin, Griffin, or draft picks as well), but the opportunity to get an All-Star caliber/All-NBA player doesn't come around often.
Atlanta's starting five: T. Young, D. Murray, S. Bey, P. Siakam, O. Okongwu
Atlanta's bench: K. Bufkin, B. Bogdanovic, AJ Griffin, C. Boucher or Thad Young, J. Williams
Why would OKC do this deal?
Without a bigger, stronger center, OKC is likely to get demolished in the playoffs by some of the teams in the west with above average, stronger centers that would be too much for Holmgren to handle (eg. Lakers, Nuggets, Clippers, Minnesota, Sacramento). Having a bigger, stronger center shifts Holmgren to the PF spot, where he will likely have a size advantage and because the opposing teams PF will likely be less bulky, the lesser physicality will decrease Holmgren's risk of getting injured and prolong his career in the long run. To make a deep playoff run, OKC also needs a capable, experienced backup point guard and Schroeder is one of the best and most experienced. Cason Wallace is going to be a very good player, but with all there draft picks, OKC can probably get a replacement/similar talent via trade/draft by next season.
OKC's starting five: SGA, J. Giddey, J. Williams, C. Holmgren, C. Capela
OKC's bench: D. Schroeder, I. Joe, K. Williams, O. Dieng, D. Bertans/O. Sarr
Why does Toronto do this deal?
Siakam doesn't fit into Scottie's timeline. He is a super talented player and with time, the current team can certainly do well once the players and coach figure out how to play to their and each other's strengths. However, with Siakam's probable max contract kicking in next year and given Siakam's age, shortens the time frame to do it in, it makes things very tight for Toronto to remain under the cap whilst filling out the roster with good players. Surrounding Scottie with players whose game fits with him is important. Younger players and increased financial flexibility is also important.
Toronto's starting five: I. Quickley, RJ Barrett, S. Barnes, J. Johnson, J. Poeltl
Toronto's bench: C. Wallace, L. Dort, D. Hunter, J. McDaniels, J. Porter