I look at it in terms of skills added.
If Tatum was going to fulfill his potential as a top-5 player in the league, he obviously needed to become stronger and more physical, allowing him to finish better at the rim and earn more FTAs. The most exciting development for him this year has been that he has become qualitatively stronger and more physical. I would say he now has the athleticism (height/speed/strength/agility combo) to be a top 5 basketball player in the world. Granted, he is still learning how to be consistent in terms of translating his new-found strength and physicality into the desired end results of FTAs and finishing at the rim, but we have seen the high in flashes. I believe the consistency will come.
Tatum's dramatic gain in this area has been over-shadowed his poor shooting #s this year. However, I don't think he suddenly forgot how to shoot. As his shooting numbers normalize, I expect we will look back on this year as another significant step forward in his development.
In a slightly different direction, I would say Tatum's development as a passer/playmaker has been slow and steady over the last several years and is moving in the right direction. It's not like he dramatically overhauled it in one off-season, but I think he could be a Kawhi-like facilitator by the time he is 25/26 if he continues on this trajectory.
So, to summarize. If Tatum's shooting slump normalizes, he continues his incremental progression as a playmaker, and he learns to use his new athleticism more consistently, I see the frame of a top 5 player in the league within the next 2-3 years. If 1-2 of those 3 things do not happen, then he will stay at his current level (somewhere between 10-20 best player in the league...a clear all-star and fringe All-NBA guy).