If I was a writer for the show, this is exactly how I would write his future. He probably would've had some relationships on the way but they didn't go anywhere. It would follow on from when Frank said to Pippa that he couldn't see himself happy with anyone but Bobby.
Alex Papps was interviewed a few years ago and when asked, mentioned along the lines that Frank would still be single if he was around.
I know that some people have said that it would be nice for him to reunite with Roo, but I just don't see that happening. However it would be nice to see Alex do another stint in Home and Away and just be really good friends with Roo.
I never watched Franks return for Sallys wedding, but based on what I have read, its a shame the writers didn't include some kind of flash back, or any mention of Bobby.