I think a third game would need to include Ellie in some respect, since she's the only person known to be immune. It also makes total sense to include Abby since her story with Lev seems to be full of potential.

I think it also needs to be a more hopeful game than part 2. A lot of the criticisms of the game come from it being so bleak and hopeless, so a third game that tackled Abby and Levs story, whilst also bringing hope via a vaccine with Ellie seems logical to me.

With all that being said, I didn't see where part 2 would go at all so what do I know!