I was initially on the fence about the suggestion of DRK dropping MRD as cross-class and taking LNC instead, but really doing a side by side of what each of those two bring, LNC is a better fit, both mechanically and lorewise.

Here is my comparison of what abilities I took from WAR and what I would replace them with from LNC and why they are a better fit.

WAR brings Foresight which I would swap for Keen Flurry.
They would both mitigate physical damage hits and parry synergizes with DRK abilities better.

Instead of Bloodbath I would take Invigorate because DRK needs a way to regain TP more than a little more self-healing.

Lastly I would drop Mercy Stroke for Blood for Blood, better and more reliable dps gain but with a risk (good for OT but dangerous as MT).

So, yeah, I would be all for the cross-class change from MRD to LNC.