I'm breaking your rule and doing guys on rookie contracts
C - Rob Williams
PF - Lauri Markkanen
SF - Tatum
SG - Brown
PG - Lonzo Ball
My thoughts, more than the names in particular...we need a PG who doesn't require shots in order to be effective, yet who has the ball in their hands a lot and facilitates an effective offense when things get stagnant. Also not a liability on defense. At PF we need a good rebounder, defender, who can space the floor (basically the Kevin Love role). Any players that can fulfill those roles I'm good with.
As for Rob, I think he could be a defensive cog if he continues to improve. I also think if the Celtics are ready to move on from Smart, they can acquire a piece like I mentioned above.
I am very similar to this. I am going to be a little lazy and not do the math on exactly the salaries but I think we would be really good with:
Center: Rob Williams or even Thompson
PF: Markkanen or Collins or Sabonis or etc.
Wing-1: Tatum
Wing-2: Brown
PG: Smart
I feel the PF role is the key. Having the right player for that role just makes everything else fit. As for the PG, I think Smart would be fine with this team. Based on having Brown, Tatum, and a scoring PF, the PG role is more tempo control and good defense. I prefer Smart for that over Lonzo Ball but I feel there are a number of players you could find for that complementary role. The Center position becomes the traditional blue collar, banger with focus on rebounding and defensive. Williams may not stand up to all that much banging but depending on who the PF ended up being, it could work.