I think part of the reason that Ainge has not gotten quality depth is directly related to what happened with Marcus Morris (and perhaps what is happening with Marcus Smart).
The Js are the future. The offense needs to be patient and feed them the ball. Too many veteran role players are too aggressive to try to prove themselves offensively, which makes the offense less patient, gives less touches to the Js, and lowers the ceiling of the offense overall. This is the main problem with many, many NBA players. They have undeniable talent, but they languish on bad teams because they feel like they have to prove their mediocre talent.
Thad Young and PJ Tucker are two guys who seem to be able to keep their usage low to accommodate players around them. I love the idea of Harrison Barnes next to Brown and Tatum, but ever since he left Golden State, he's been trying to prove he is a go-to scorer, and the teams he has played for have struggled because of it. If he was willing to limit his shot creation (we could use some, but not as much as he has grown accustomed to) and focus on defense and corner 3s, then he'd be perfect.
If the Celtics were able to pick up a player like the Warriors did when they got Andre Igoudala, that would be perfect. What player out there helps his team win games, but clearly isn't comfortable shooting as much as he currently does, and would love to play around high usage guys?
Ironically, I thought Gordon Hayward could be perfect. I do think Kemba Walker is a good natural fit. SGA is one guy who came to mind. He reminds me a lot of Andre Igoudala from his Philly days. He's probably underqualified to be a franchise cornerstone, but he'd be a great fit as a low usage guy and switchy defender. But OKC isn't trading him yet. Malcolm Brodgon fits this mold of player, but even with Levert out, I don't think Indiana will become sellers. Jrue Holliday is probably the closest to this type of player, but the Bucks aren't selling unless a catastrophic injury happens. Ben Simmons is another guy that comes to mind, but he is not really within reach and is probably a little overqualified for what we are talking about here.