As per the title, what recommendations would you give to make Frost DKs competitive in PvP that is consistent with DK flavor?
To start off, in terms of survivability, I think Frost DKs armor being covered in a magical rime allows them to shrug off damage - 10% passive damage reduction.
Also, as far as mobility goes, it seems strange for an ice covered juggernaut to be particularly mobile, but rather it would be interesting if their mobility came from denying mobility to others. Like maybe a pvp talent that made it so that movement is capped at the Death Knights speed while within an X yard radius, and while Remorseless Winter is up, it cancels the first displacement effect used within its radius.