His parents are dead, so Voldie could adopt him (with Bella presumably assuming that Dolohov would happily do the rites) and then possess him. Presumably he'd also need to AK somebody else, which I'm sure Bella would happily volunteer Dolohov for...

...or possibly the plan was just for Voldie to AK Rod so he could get back out of Harry's body into his old one?

Or maybe Bella really did think Rod could offer assistance, who knows.