It seems to me like this is kind of the problem.
What is actually fundamentally different about dual-wielding frost and Arms, if the difference between 2h frost and Arms is so little? If they're so similar that changing their itemisation slightly demolishes the difference between the two specs' identities, it seems like they're already the same class.
2h frost seems perfectly fine to me; open up new options for itemisation and aesthetics with little downside. Sure one will come ahead on DPS but there'll always be a top DPS.
Part of what pickles me about this case is why Frost mirroring Arms Warriors is the issue. Why aren't dual-wielding Arms Warriors mirroring Frost DKs? What about 2h Fury Warriors? These are things the game had before, and could be differences that help to make your character progression more meaningful.