Actually, one left field option (for pairing with Cousins) would be Brandon Bass. People will probably think I'm crazy, but hear me out. I have a few reasons behind this choice:
1 - He can hold his own defensively
Demarcus Cousins is one of the most talented guys in the league, and on ability alone he's easily a top 5 player in the league right now - a genuine franchise talent. If he's on your team you want him on the court as much as possible, which means you don't want him getting hurt, tired or into foul trouble.
Despite being undersized, Brandon Bass is surprisingly capable (and versatile) defensive player. He has the agility to defend more athletic bigs (e.g. Thaddeous Young), the length to defend taller bigs (e.g. Brook Lopez) and the muscle to defend more physically imposing bigs (e..g Zach Randolph). If you can put Bass n the opposing teams best offensive big, that takes pressure off Cousins so he can dominate offensively.
2 - Stretching the floor
Bass has always had one elite skill...his midrange jumper. For about the past 8 years he's been one of the of the best midrange jump-shooters in the NBA, and because of that reputation he's rarely left alone on offense. DMC is going to attract constantly double teams, but any team that leaves Bass to double Cousins is going to pay for it about 80% of the time. This is becoming an even bigger factor now that Bass is learning to extend his range to the three point line.
With someone like Cousins drawing in all of the defense, a guy like Bass could get a LOT of easy baskets and would be realistically capable of averaging 14-15 PPG.
3 - Physical play style and second chance points
Both Bass and DMC are very strong, incredibly physical players. Those guys aren't easy to push around, and they will wear you down constantly with their physicality. Both are also outstanding offensive rebounders, so if you surround them with shooters you're going to get a ton of easy baskets off offensive rebounds.
4 - Attitude
Cousins has been constantly frustrated in Sacramento because he feels nobody takes the game as seriously as he does, and nobody puts in the effort he does. Also he's mostly played with other young, inexperienced guys. As a result he's lost his patience many times, and has developed a reputation for having a bad attitude.
If he played alongside Bass on a nightly basis, I think that would change.
Bass is a savvy veteran who has played key roles in some big playoff games, has played with hall of famers, he's played on teams riddled with injuries, played through a variety of roles (from starter through to bench warmer) has just survived through a rebuild. He's pretty much seen it all, yet at 27-28 years of age he's still young enough to be able to relate to a guy of Cousins' age.
More importantly though, Brandon Bass is a blue collar guy. There might not be a single guy ont he Celtics roster who plays as hard as Brandon Bass on a nightly basis. He puts in 100% effort every single night, he's holds his own on both ends of the court, and he's tough as nails (yet still incredibly calm and grounded). He really is the ultimate professional.
Not only do I think DMC would develop a huge amount of respect for Bass, I even think he'd look up to him as a mentor. While a guy like KG might might be too intense and emotional to get remain patient with someone like Cousins, Bass is calm enough that he could probably keep him grounded.
Bass is by no means the most TALENTED player to pair with Cousins, but he might be one of the most suitable.
Actually, all of this is actually a big part of why I think Boston would be such a perfect fit for Cousins. How can you NOT have respect for your team when you are sharing a locker room with guys like Marcus Smart, Avery Bradley, Brandon Bass and Brad Stevens - all while being constantly surrounded by all that the Celtics history and tradition.
If that's not enough to transform a guy like Cousins, nothing is.