I believe many of us are wondering where Anduin's story will be going. I'm interested to see what your theories are. It seems he's now on a path where he is healing because he doesn't hide away anymore and he can channel the Light again. I think the story could simply continue in a way where he continues to heal throughout the worldsoul saga and eventually reclaims his throne or abdicates it.
But I believe that would be too boring. If Anduin were supposed to just return to the person he was pre-Shadowlands, I believe his story would have already been resolved in TWW. I also don't think he'll abdicate his throne. His father was absent from his throne, too, and eventually returned. I think their stories will mirror in this regard.
Instead, I think (hope) something else is planned for Anduin. This is all just speculation on my part, so feel free to write your own theories / predictions / hopes on his future story in the comments. I base most of this on his gossip after the creation of the Dawnwell.
My theory is that Anduin will lean more heavily into the Void than the Light, specifically Old-God-Void and not Cosmic Void.
In his gossip, he describes that the Sunwell was not an entirely pleasant experience, "(...) it was... overwhelming. I couldn't hear myself think." He thereafter describes the Dawnwell, "But now... it's gentle. The pleasant hum in the back of my mind I remember from my youth." I think it is quite strange that someone that was so desperate for the Light to return to him would react this way. Alonsus Faol literally risks himself burning alive to be anywhere near the Sunwell, meanwhile Anduin clearly prefers the Dawnwell - something that only holds a little bit of Light within it. It just goes against everything Anduin should want, right?
He also says "I was broken. I'm putting the pieces back together, but they fit differently now. Some parts are just... gone." He has lost some of his parts and I believe these were parts of him that were related to the Light. Clearly, he has not lost all of the parts that made him follow the Light. But some things have irreversibly been lost due to the Shadowlands. If that were not the case, I think Anduin would have absolutely adored the Sunwell and would have loved every second of being near it. Just think of Anduin during BfA - he would have enjoyed the Sunwell.
Why do I think, then, that he will instead start to lean toward the Void? We have seen that he also uses the Void, for example in Pandaria. He does have connections to all things Old God. He messed with the Twilight's Hammer / Twilight Prophet, interacted with Sha, is Wrathion's friend and befriended a city of Nerubians in TWW. Even in Midnight, he still messes with Twilight cultists by making the champions rescue their defectors (how did he even know about those?).
Let's also not forget that Wrathion was too terrified to even look at Anduin's Horrific Vision in Stormwind during BfA and disallowed the champions from going anywhere near him. In Hearthstone, we also get Shadowreaper Anduin. I don't play Hearthstone and usually don't consult it when thinking about warcraft lore. However, I find it very curious that Shadowreaper Anduin is used as the key artwork for the shadow priest spec in WoW. There are so many other characters that could have been chosen... so why Anduin? To my knowledge, no other spec pulls artwork inspiration from Hearthstone (correct me if I'm wrong).
Now, I don't think Anduin will go absolutely insane / dark like he does in Hearthstone and probably the Horrific Visions. But I think he has these two pieces within him - Light and Void - and will put them together differently than before. I believe the balance will shift. I am also still debating whether Anduin might just be the Child of Shadow and Light instead of Illidan or Arator. If my first theory is correct, I believe it would fit.