Ninja - What abilities might it borrow from other classes?

[Nahara]

Most likely, it'll be Pugilist and Lancer. That gives you the four really big cross-class abilities for melee DPS: Invigorate, Blood for Blood, Second Wind, and Internal Release. Marauder could possibly be one, as Bloodbath and Mercy Stroke are fairly nice in the hands of a DPS, but not as nice as those four.

I don't see Archer happening, as SE seems to want Raging Strikes and Quelling Strikes kept to ranged DPS and casters (which is fine, as they don't really have any DPS cooldowns on their own). I REALLY don't see Conjurer, Thaumaturge, Arcanist, or Gladiator happening; none of them give you anything that would in any reasonable way benefit your job as a melee DPS. Gladiator gives you tanking cooldowns, which you don't need. Conjurer gives you healing spells, which you don't need. Thaumaturge gives you Surecast and Swiftcast, okay, but if any of the Mudra actually have a cast-time, the ninjutsu better hit like a train, because that's a LOT of DPS you lose by casting them (and Blizzard II is just useless to everyone). Arcanist gives you a mixed bag of stuff, but still, it's useless for you as a melee DPS.

I feel as though they may cross class skills with MNK and DRG. And DRG and MNK may very well lose MRD skills and get NIN skills if they are more valuable.

Essentially this. The most likely thing to happen is NIN will receive cross-class abilities from the Pugilist and Lancer, while the DRG and MNK drop Marauder from their list of available cross-class abilities and receive Rogue's in exchange.