There should be little doubt that Sexton will bust his ass this offseason and come back a better player.
I’m just curious as to what his ceiling is?
I don't think you really know with a guy like Sexton until a few years pass.
Can the right coaching staff work with him an defense so he can become average or better than average? Sexton was 101ist (2nd to last) in defensive Real +/- this year at -4.61 (only Trae Young was worse). De'Aaron Fox was 0.49. Fox was near the bottom in his rookie year at -2.71. That's a big gain. Can Sexton make a similar or better leap? If he shows dramatic improvement in a single year, that raises the possible ceiling for him on defense.
Can he pull a Kyle Lowry over time? Lowry averaged 8.7 assists per game this year. He averaged 3.6 assists per game in his first full season. Granted, he played fewer minutes than Sexton. But after a few years, Lowry doubled his assists per game. Will Sexton show signs of growth here? April was a short month and he did have a 10 assist game, though even without the 10 assist game he averaged 3.5 per game in April. That was an improvement over his seasonal average. There's an upward trend. Can Sexton jump up to near 5 assists per game next season? Like on defense, this would raise his possible ceiling a bit.
There's also a question as to how much will Sexton continue to grow. He may add an extra inch or two.
If Sexton doesn't take a step back in any facet of his game next year while improving on weaker areas, it'll still be hard to peg his ceiling. I tend to believe he's not your Batman, but I think he has the potential to be a very good Robin.
If his defense improves, he still shoots around 40% from 3PT, scores 20PPG, and manages 5APG while limiting his turnovers - I'd say we could feel confident in saying we have a pretty good player on our hands.
Add in a genuine star from this draft and likely another top ten pick next year plus the 26 pick and any other picks attained via trading ending contracts - and having a hopeful outlook for the future is pretty fair.