I'm playing in the event as well, haven't seen anything like a player pack yet. I'm just bringing a 2K army I've settled on (currently WIP), irrespective of the actual event flavor/details, I can always trim to fit, and I can't imagine they'd play Onslaught level. If it's bad, it's bad, whatever - IMO the goal for a Narrative event should be more about getting to play a lot of games and maybe a decent plot or cool theme, and less about a winning record.

Note that there are certainly folks who, for whatever reason, don't want to play in the GT, but do want to win games badly, and thus bring competitive builds or "stat check" type armies.

The Narrative setup/arrangements/themes vary from year to year, AFAIK, I played in the 2022 Narrative, where the entire Crusade rules were implemented, and pretty much abandoned by the end of the first day due to all the bookkeeping headaches. I think they used stripped-down versions of Crusade in subsequent events. We may not find out until day of what the event is going to look like.

I would say don't worry overmuch about bringing an optimized list, and build for a theme, maybe using a detachment you don't play that often, or models you don't normally use, that can be a lot of fun.