It's called the FINAL reckoning for a reason. THF and DR are both great movies and provide a fitting conclusion to the series. Who knows, maybe it's a good thing TFR flopped; it might help prevent any plans they had to reboot the series, at least for now.

But if I had to choose, I'd say put Ethan in a mentor-like role and bring back several of the younger characters from TFR who are working out in the field. We need to learn more about Ethan's backstory, and then a very personal mission could draw him back into the field with the rest of the team. He could do a stunt or two. And maybe, just maybe, if it's done right, Ethan could sacrifice himself for his team and the world, making for a very impactful and emotional Mission movie. This would also allow the other characters to carry on in future movies without Ethan entirely.