Luffy's bounties have in the past, tripled or more. Him getting a filler bounty with an increase of "only" an extra 100 million suggests that yes, he'll be getting something ridiculous.
Between hitting the Tenyrubiito, Impel Down, the war, and connections to the revolutionaries, the announcement of his father being Dragon, and displaying potential Ambition abilities, it actually kind of surprising he didn't get a higher one. But I think that can be attributed to 1) the credit being spread around and Buggy claiming most of it, 2)him getting his ass kicked repeatedly during the war, 3) the 2 years of silence and presumed dead-ness.
Now that he's back in action, and his crew is reassembled, and he's going to be showcasing full Ambition ability, and apparently destroying Fishman Island, his threat should be raised considerably again.
Hence, the placeholder bounty. It jumping only another 100 mill to reach 500 mill would be silly, there'd be no need for the placeholder then. It probably is going to be reeeeally ridiculous, Urouge's guess of 1 billion probably isn't too off the mark. (Usopp hinted at such bounties existing in the billions when they first met Aokiji.)
Put this in perspective. The world nobles were willing to toss out 500 mill on a whim for a mermaid. What do you think they'd be willing to put out on someone who HIT THEM. Based on that ALONE I'm amazed Luffy's bounty isn't higher. (Its also possible they have a separate reward set up for him in addition to what the government is offering.)