Middleton would have been especially interesting had Barnes been traded. He's another vet with championship experience, and he likely has a good relationship with Keldon Johnson due to their Olympic run. Even with Barnes, having a similar player to him wouldn't be bad. It doesn't fill the holes in Barnes' game, but it does provide insurance in case Harrison struggles in a game. I also think he'd still be good for Vassell, Castle, Harper and Bryant to learn perimeter defense from.
I've been a Bagley fan since his draft, so I'd be down with the Spurs signing him. The issue is that he doesn't shoot well, which means he's basically a center. He's still spry and agile, so he might be the big who'd fit best with Wemby. But I think the inability to put another big next to Kornet is the bigger handicap. Luke only had average mobility, and that means he has to choose to give up a lot of ground during perimeter action or leave the paint undefended. Having a power-forward with legit weakside rim protection would allow Luke to commit to stopping penetration, which means fewer breakdowns down the line.
Ideally, the 15th man could do a few of the good things both Middleton and Bagley do to at least some extent. The issue is that guys who even have a chance of being that kind of player are worth their weight in gold. Where's Ryan Richards when you need him?