Ultimately, my tune might change on Fox in the summer, but I'm definitely out on him for a move now (not even sure he's truly available now). There are just too many questions around what we already have to make that kind of move, notably two big questions, which happen to be of extra importance relative to discussing Fox: 1) Is Devin him? and 2) What is Castle?
Fox will almost certainly cost Devin or Castle (Kings fans seem to want both, which ain't happening). The good thing about Castle is that he offers flexibility and could theoretically play very well with Fox or Devin (though the pairing with Devin is a little more theoretical as we've seen more of Castle as a ball handling SG than a lead PG). If Castle could be a true lead PG, and Devin could prove consistent and healthy, then I'm not sure I want to use assets to disrupt that pairing - I'd rather save my chips to make a move for the ideal wing archetype (Markkanen, Murphy III, Cam Johnson, Jalen Johnson, MPJ are names that all fit, IMO - not all will be available, and they all carry varying costs).
A Fox-Castle pairing is easier to envision because we've already seen Castle in this role and it's easy to understand what he brings and what needs to be developed. SpursBills laid it out in his post, and I echoed. You'll need to match these two up with sharp shooting 3&D wings. Champ and Barnes are okay for now, but you'd want to upgrade those. If Castle develops into an average shooter, you still need those types of wings because Fox is only an average shooter. But your backcourt defense is improved over having Devin in there.
A Fox-Devin pairing lets you play Sochan, who can pick up some of Devin's defensive shortcomings with his perimeter defensive skill, and Devin adds a lot more spacing versus Fox-Castle. I think this actually be one of the best pairings for Devin, honestly, because a lot of ball handling and playmaking would be taken off his plate and less reliance on him attacking the rim, and he'll be able to be more of an opportunistic slasher going towards the rim while putting his jumper to great effectiveness.
A Castle-Devin pairing, which we all assume is the default setting, carries all of the unknown variables that I'm hoping we get answers to this season. Can Devin become more consistent? Can Castle function as a lead PG for extended periods of time? I'm hoping we get some significant Castle-Devin-Champ-Barnes-Wemby lineup minutes to see how it looks. So far we've only seen 4 minutes of this lineup and 5 minutes of the lineup with Bassey in for Wemby, 4 minutes of it with Collins in for Wemby. I need to see more Castle-Dev-Champ-Barnes.
Honestly, the player who fits the least in any of these scenarios is Sochan, who I have been really been pleased with this season and am starting to like a lot, but I think his best role might be as bench glue-guy if Castle is going to a major player for us. If Castle develops into an average shooter, pairing him with Devin is Jeremy's best pathway to be a starter, because I've lost any hope Jeremy will develop into an average shooter.
It's most likely going to cost us, at a minimum, Castle or Dev, 2-3 FRPs, and their swap back. I'm not concerned about the contract - that is easy to work around, especially right now. I am more concerned about what other deals this might crowd out.
I do think we need to start moving some picks, because while "there is no rush" - the are starting to come due. I don't want to punt them for anymore 20131 picks, because those are also less valuable as trade chips. With that said, I'm not even sure we need to make a move for a bona fide "tier 1" star (whatever that means and whomever is in that category). A player of Fox's caliber (like a Markkanen) is a good fit next to Wemby, especially if Castle develops like we hope. But Fox specifically still leaves you having to fill the same holes you have now on the wings... I'd rather get a Malik Monk-type player to be my 3rd guard, and "blow my load" trying to get someone like Markkanen.