25 is still very young with players not completely physically or def not BBIQ mature yet. I have the peak age to be 28-29 for most players.
25 is young but most players are what they are around 25, minus the incremental improvements. Those incremental improvements add up over a span of 3-4 years though, so you get the peak age of 28-29.
Jaylen Brown will probably improve on D and decision making. His handle might improve slightly too.
With that said, Brown has some weaknesses in his game and it's easier to improve from bad to average than from good to great That might give him more opportunity for growth than others, but you'd have to think he has been working on these areas all along. If that's the case, there really isn't much room for growth. There is a very good possibility Jaylen has been working on his dribble every summer, and this is the result.
I'd guess if Jaylen makes any real leap, it will be on the mental side. Or whatever you want to call it. I hate the word mental. Better decision making, learning when to make an easy pass instead of trying to drive thru 4 guys. A better understanding of defense thru experience etc.
I could see him maybe making All NBA 3rd team at his peak, especially when I look at the list this year but I don't see him ever being more than a Robin/2nd banana.
Brown will be more or less the same player next year, and I'm totally cool with that. A Jaylen Brown with an above average dribble is in the 2nd tier of players. Or possibly 3rd if you have a small 1st tier, but 11-15 range. It's nice to dream on but I don't think it's likely. And I'm not sure Jaylen really proved himself to be an outlier in terms of development. His development is pretty normal for the players who do succeed. He didn't have the Jimmy Butler or FVV development path.
if he does make another big leap, the C's will be looking at not 1, not 2, not 3... This is especially true if Tatum and TL have any type of improvements over the summer. Considering it's their age 24 summer, there is a good chance of that. Hell, the team is already going to be competing for titles every year as it is.