Earthbound BUT Frank Fly is one of the members of the player party now.

Maybe joining Ness after defeating him?

That's exactly what happens in the prequel. The 4th party member is a knife-brandishing gang leader that joins after you defeat him.

No deadly robot assistant, though.

Now that I think about it, you'd be kinda overpowered, since with Frank you'd have both the robot AND the Sharks to help you.

It'd be a great ROM hack! The robot could follow like a teddy bear and tank some hits, and the Sharks could follow like the Flying Men.

But we'd have to lose either Jeff or Poo too. Every day I think of a new defect to this idea :(

About the said Teddy Bear mechanic: How does one have more than one Frankystein Mark II?