Imo, Tatum is at his best when playing at the 4. Traditional 4s aren't explosive enough to stay in front of him on the perimeter. This is a massive advantage in our favour, especially since Tatum can put the ball on the floor and create his own shot.
Likewise, imo Brown is at his best when playing at the 2. He's mobile enough to stay in front of explosive guards on defense. At the same time, he's taller than most SGs, hence he can shoot over them or take them to the basket.
Generally speaking, the C's are built around two switchable wings in Tatum and Brown. We wanna maximize their effectiveness on both sides of the ball, hence we run a switch-heavy defensive scheme. With this in mind, here's what I'd be looking for.
PG: Oversized, pass-first PG who's happy to play off the ball on offense. For instance, Cade Cunningham, Lonzo, Brogdon, Jrue Holiday, Satoransky, prime George Hill. Smart fits the bill as well. Not exactly a pass-first player, but he's a willing passer, that's for sure.
SG: Brown
SF: 3+D swing. For instance, OG Anunoby, Joe Ingles, De'Andre Hunter, Otto Porter Jr. Alternatively, a Point Forward. For instance, Hayward, Middleton, Kawhi (he can do everything, literally the perfect fit), Kyle Anderson, Joe Ingles (fits both descriptions), Harrison Barnes (not exactly a Point Forward, but certainly an above average passer for a swing), Tobias Harris (again, above average passer). Theoretically, Fournier is expected to fill this role. Personally speaking, I don't like his defense.
PF: Tatum
C: Tatum scores plenty of his points out of the PnR. We need an elite screener to set him up on offense. Ideally, an elite roller/popper as well. We also need a switchable big who can play D on the perimeter. Some names that come to mind: Bam Adebayo, Christian Wood, Ben Simmons (yes, at Center), Jarrett Allen, Clint Capela, Richaun Holmes, Daniel Theis, Khem Birch, Onyeka Okongwu, prime Horford, prime Ibaka.