I think it's just a shame that the Mitchell unit has completely disappeared. No Ronnie, Grant, Sam, Peggy, Ben etc. Shirley wants nothing to do with him, Jay has moved on...
I'd actually like to see his daughter Louise return but the only way that could happen would be if Lisa died off screen but that'd be too coincidental to the whole Kathy/Ben situation. Maybe in a few years when Louise is 16 years old, she can come and find Phil and then gradually, they could build up a strong bond and Phil finally does something right with his own family. Other than that, what is left for Phil?
Maybe when Roxy and Alfie inevitably split up, maybe she moves in with Phil?
You forgot Billy
I think the last time there were very few Mitchells they created the sisters but apart from another brother of Eric that may of had children, there is no real way to expand the Mitchells further and that would just seem a recycled step.
I think Courtney will come back without her father at some stage and stay with Uncle Phil. Roxy and Phil have never really connected - Ronnie was the glue between them really (Phil knows Ronnie has the brains and Roxy is just a thicko)
It is a shame but Phil could survive on his own - Ian and Dot pretty much have at times.