Lucy Hare should be the 5th Star Fox member
(Image Source: https://x.com/monchirohana/status/1288860194420559872?s=20)
When James died, Fox became an orphan. It was Peppy who took him under his wing until Fox was ready to follow James's footsteps. During this time, Fox should have probably formed a sibling bond with Lucy.
During the events of Star Fox Command, we learn that Lucy always wanted to become a Pilot. We also learn that she became Krystal's best friend.
This last bit is specially important. One of Krystal's biggest flaw is that she's written to be Fox's love interest, and has little to none interaction with other Star Fox members. Having her BFF in the team prior to her return will allow Krystal to have some decent interactions that do not revolve solely on being Fox's girl.
Being Peppy's daughter, Fox's symbolic sister, and Krystal's best friend, Lucy has a lot of potential as a mediator that helps to improve the relationship between members of the team and avoid infighting during rough times. Her inclusion to the team would be easy and would probably feel natural, since Slippy and Falco should already know her by now.
If you are still not convinced, imagine an alternate dimension/reality where Peppy dies in Venom and James is the one to make it out alive. Being now an orphan, Lucy is taken under James's wing, and both Lucy and Fox are trained by legendary Mr. McCloud himself. Can you imagine how badass these two would turn out to be? Surely that sounds interesting at least.
I certainly believe Lucy should be added as the 5th member, I'd love to see some brother-sister dynamic between Fox and Lucy. As much as I love Fay, Miyu and Krystal, I believe Lucy has the biggest potential to make a good narrative, at least in the early stages of the franchise.