Forrest

I think Zo and Varl's child would be the best choice for a new Horizon series or even a one off story. There is a lot of potential there for some good plot lines.

I also think Seyka and the Quen would make a good story. Considering what we know about the Quen and their Emperor, Seyka could be a catalyst for change with in that tribe. However, I do hope Seyka is in part three and aids Aloy against Nemesis.

Another way WB could go would be to follow the pattern Gearbox set up with the Borderlands series. While Borderlands can and is playable in single player mode, it is really intended to be played in co-op mode with up to 4 players. Add in some DLC and HL2 could still be a hit.

One way around this is to make the character in HL2 take a position other than seeker. Keeper would be the "easiest" as you, the player, would be in a semi fixed position near the goals and only have to keep the quaffle out of the rings while dodging bludgers. If players need a choice give them anything else but seeker.

If TLOU3 is done like the first game then I'd be ok with that. In TLOU2 we only played as Ellie in the single dlc. For the rest of it we played as Joel. At this point in the story Abby has to tell her own tale. She shouldn't have to share the spotlight with Ellie. IMHO the only story that might benefit with two protagonist is Joel and Tommy's during the 20 years prior to TLOU2. Even then it probably shouldn't be done.