Well Mangold did say
...I have taken finger to key. Let’s say that. There’s been typing. And ideas. And talking amongst all the principles.
Which I assume means main cast/principal cast. Hugh Jackman + ?????. Which I assume means people that have been featured in an X movie before and not new hires since there's no casting going on and no pre-production yet. A lot of assumptions here. It could mean the major business players instead.
And here's a twitter convo featuring Mangold and Schreiber (Victor Creed)