I'd rather an unknown be cast - and if they're going to reference Ripley and Hicks being older (which isn't a given), then they'd have to cast a teenager or twenty-something actress.
But, yeah, I'd rather an unknown actress be given a chance. Plenty of them out there.
And absolutely not 'J-Law'... The woman is f**king everywhere.
Hell, if something's gone funky with Ripley's/hicks' cryotubes, then they can just as easily set it many decades later and say Newt's was even more affected. Rebecca could be of a similar age to Ripley. I'm not sure that would be a wise decision, though... Mentally, she'd still be a child.
Of course, if they're using the original comics as a template for this, then Rebecca could have been placed in care, only for Hicks to discover she's been treated very badly. It's ironic, but it's essentially the same storyline Linda Hamilton performed so well in 'Terminator 2', many years later. But, yeah, she could be a hard-edged, strung-out traumatised train wreck, not too dissimilar from how Ripley was at Gateway.
And, honestly, considering the director's preference for social commentary, I could see him exploring some kind of narrative on poor standards of mental healthcare. It's there if he wants to use it.