The shooting is the obvious issue now that he's getting schemed, but it looks like his pathway to success are leveraged against 3 main things:
1. Shooting (the Jrue Holiday) - if Castle becomes a Jrue level shooter throughout his career, I'd be very happy with this at this point. 35% on medium volume, elite defense, not quite a full time point guard. For this to happen, he needs to improve his FT%, 3P%, and just his shooting profile in general. Jrue throughout his career was at about 79% on FTs. Defense-wise, I think it's a much smaller jump that he needs given his size strength and physicality. This is not a max-level player, but an elite role player nontheless.
2. Turnovers (the Jimmy Butler) - frequently cited as Castle's ceiling, but Jimmy's underrated superpower and the reason why he's an analytics monster was the ability to sustain high usage rates with a very low turnover rate in spite of being a poor/non shooter. From mid-career on, Jimmy was able to sustain mid 20s usage rates with a turnover rate < 10. Castle's at 26% usage but 19% turnover rate currently, which is unacceptably high for an on-ball guy. His foul-drawing, physicality, and defense are already there, but that turnover rate is going to be what separates him as a legitimate on-ball guy. This is probably a max-level player, even in today's cap environment
3. Ancillaries (the Thompson twin) - these guys are straight up non-shooters, their handle isn't as good as Castle's, and they don't really seem to be able to run an offense that well. However, they're basically maximizing value from possession-maxing ancillary abilities with elite offensive rebounding and turnover generation relative to position. If Castle isn't able to minimize turnovers and his shooting remains problematic, this is a third avenue of success for him, although this relies much more on leaning into his wing skills. This is also not a max-level player, although it will be interesting to see the type of contract the Thompson twins get
The most likely thing to happen is that he'll improve a little bit from buckets 1, 2, and 3 but not lean fully into any given pathway.