There is little that I both look forward to and dread more than the thought that Phil may one day decide to play Hollow Knight, which only seems more and more likely now that all of its free content updates have been released, knowing that there's so much he can just completely fail to understand. I also await whenever he decides to go back to do Metroid Prime 2 and any other Kingdom Hearts games, especially Birth By Sleep. I've also been holding out hope that he might go back and play Salt & Sanctuary again, Souls-type games are always good for a laugh. Same for Night in the Woods but then he did end up finishing it, it turned out surprisingly tame by Phil standards too, probably because in even the handful of actually difficult gameplay sections you still don't get barred from progress for being bad.
A guaranteed gold mine with Phil is any game which he claims to have played in the past and been a fan of, especially old school games. I know he's said at some point recently that Super Mario RPG had been one such game, I'd love to see him try to flail around at that. An Earthbound playthrough would be great too but I saw that he's already played that before and apparently it didn't get a lot of attention so he's probably not going to be willing to try it again. Maybe he could get the Mother 3 fan translation.
I've seen a few interesting suggestions in this thread like LISA and Megaman Battle Network, sadly I don't think those older or more out there games are too likely, though if I were to hold out hope for something obscure myself it would probably be Gotcha Force. I'd note that I believe during Phil's Super Mario Sunshine playthrough he mentioned that the door's open now for any Gamecube games he didn't play during his oft-mentioned fighting tournament days, and one of the things he specifically mentioned was, of all things, Star Fox Adventures - now that would be hilarious.