I don't think
Matilda or
Arthur are particularly likely choices for a future monarch, though for subsequent children though they'd be right on the money.
King Arthur would be too cartoonish, and
Queen Maud is too remote a namesake. Also,
Matilda is trendy in the
UK so I don't think it'd have enough gravitas right now to be considered.
For a boy, I'd be astonished if it wasn't
George or
Albert.
For a girl, I'd look for
Victoria,
Alexandra, or even another little
Elizabeth. I'd really love to see
Eleanor, but I think it's probably also too remote.