It's hard to predict.
Tatum ceiling = KD or PG13
JBrown ceiling = KLeonard or PG13
...right now, JB is closer to KL/PG13 than JT is
I don't get the comparisons for Tatum. He doesn't have the length/size of Durant, though he could maybe develop a similar arsenal.
PG13 is one of the best defenders in the league, while Tatum's defense has been Melo-esque. That doesn't mean Tatum can't fix it - James Harden has actually been playing decent defense this year so
anyone can change - but I'm just not seeing the comparison.
I think his ceiling really is Melo - which isn't the insult many may think it is. People clown on Melo now because of his last 5 or so years, but prime Melo could score from anywhere and was crafty as hell.