Spilotro would have, without a doubt, become the next Grand Avenue Crew Don had he not gone out west. Or at least the underboss under Louie Eboli. It was basically Lombardo, Eboli, Spilotro, they were Lumbo's top two most favoured men. Because of Spilotro's brotherly relationship with Lefty Rosenthal, it was decided that it would be best if he became the Vegas capo. Eboli would become Lombardo's personal underboss.

So no, he was definitely somebody, and the definitive #2/3 under Lombardo. He wasn't gifted the Las Vegas rackets for nothing. There were a ton of guys vying for that spot, and a lot of men resented him being given the position.

At the end of the day, Lombardo's biggest mistake was allowing Spilotro to put together his own crew. It's difficult to keep tabs on a guy when he's so far away, especially back then.
On a side note, I know in Casino, Scorcese had the Spilotro character reporting directly to the Joey Aiuppa character (Remo Naggi??). In reality, Spilotro reported directly to either Lombardo or Louie Eboli, and they would in turn report to Aiuppa.