Right now my ideal for Part III is just about Ellie and her growing as a person from the ashes that we were left with at the end of Part II. Some major points for me:
1. I don't see Ellie as OK at all at the end of II. To me she's at most abandoned the path of vengeance and hate, realizing it just leads to more pain and loss and recognizing that she is capable of forgiveness and there is power in it. But beyond that Ellie seems pretty rock bottom, she's still have major survivors guilt. She's maybe accepted Joel's death, but I wouldn't say she overcome it. Nor has she really grappled with what she did in Seattle and everything that happened. The silver lining to her being at rock bottom is that means things can't get worse and she can now recover and redeem herself.
2. I'm not sure Jackson is a lasting home for Ellie, or at least somewhere that feels like home to her right now. Part II didn't give off the impression she really fit in there or was very close with anyone outside of Cat, Jesse and Dina. As well as Joel and Tommy. Joel and Jesse are dead, her relationship with Dina is all sorts of messed up and really strained, if not over, due to the events of Part II and Tommy has become a real angry and bitter dude. Who I doubt will get over things after she returns from Cali having left Abby alive. Whether she leave immediately or in time who knows, but I think finding a place in the world literally outside Jackson as well as figuratively could/should be an element of Part III. But I wouldn't be opposed to her and Dina coming back together in the end.
3. Finding a cure should not be an option. Joel's actions have to have lasting consequences for Ellie and the whole world. That Ellie wishes for it to happen to give her life and survival meaning doesn't make it worthwhile, if anything it's a reason not to have it happen. As much as the spores are a threat to humanity, humanity is more of a threat to itself. Ellie needs to find a reason and purpose in life beyond sacrificing herself and come to terms with her immunity and the fact that it cannot save the world and that she survived while others haven't. All that she can do it make something of her life for herself and those she cares for. And the rest of humanity needs to come together and find a way to live and cope with this more hostile world.
4. Abby and Ellie have no business meeting up again, especially as the cure thing is dumb and pointless. There is no realistic way for Abby and Ellie to ever meet up again beyond that, let along team up and have it work out. There's a huge gulf between ending the cycle of violence and vengeance and not wanting/needing to kill one another and actually being OK and working with each other. Abby and Ellie did horrendous things to one another. In no world, fictional or not, should those two be forced to work together. I think Abby and Lev could make for a decent stand alone expansion, but on the whole there's really no need or use for them in Part III that focuses on closing out the story of Ellie.
5. Ellie should not get some child/youngster side kick that gives her hope. Especially not JJ. It's a great trope, but it's been done twice now in the series. First with her and Joel and again with Abby and Lev. It's very effective, but three times is way too much. They need to be able to present a story and redemption of a character without the crutch of a younger person to help them finding meaning again in life. It would be very tragic if that's the only way they can think of to inspire hope and purpose in a character's life again. Ellie needs to find something else to help bring purpose to her life and to care about.
6. Having something happen to Dina, JJ or even Jackson as a whole to kick off things and be some call to action would feel cheap and easy. After the explosive and dramatic start of Part II it would be worthwhile for the events of Part III to take a different and more mundane approach. Relatively speaking. Have Ellie out in the world for one reason or another. She left Jackson behind completely, or she's distancing herself and going on long patrols or courier missions between settlements on her own. Have her get badly injured either by happenstance or from an attack and saved by some new community who nurses her back to health. Make it a kind of fresh start where Ellie helps them in kind and from there starts to build relationships with various individuals and the community as a whole. Start off simple and small and then work up to some bigger conflict with outside forces where Ellie finds she really cares for these people and this place and wants to protect it. Have it so that it's not just one person like a child or love interest that saves Ellie, but instead humanity as a whole, a community of people that she cares for and feels loved by and wants to protect and see survive and thrive.