At least form my experience in 1.3, the excalibur is a good example of an Initial: Haste weapon that works. Why, because it can't be used with Two Swords, it can't be used with a shield, and it must be used on a really slow class. If you ever add a Ninjato with that, you'll essentially add more speed to a class that is known for its speed. But for a knight or a lancer, it is a lot more on pair. It essentially balances their speed for the beginning of a battle.
Shadow Stitch. Stop proc is stupidly strong. Wield two of those, and you have a 44% of hitting them (if the proc is 25%... if it is 20%, then it's 36%, still pretty good). You'll make Ninjas not only be superior in terms of speed and raw damage, but they'll stop any enemy they want in a 1/3 ratio. Now, what if you used five of them...
Initial: Protect and Initial: Shell ain't so bad. The +MA already isn't too relevant for a ninja, as seen in Spell Edges in 1.3. The +PA could be very strong by itself, but I'm not sure. It could work.
Weapon Break proc. I'm not sure how this would be, it sounds bizarre / niche. Maybe with a low WP it could work well. The thing about it is that Knights don't have much as they are (if you're not modifying them from 1.3's skillset), and Weapon Break, along with Power Ruin and Mind Ruin are the 3 most relevant abilities they have. I'm not sure, but I'm thinking Break Shield would be a bit more varied. *If* you go past the unit's evasion, there's a proc you'll break their basic evasion item. And a Ninjato breaking a shield sounds cool.

Koga and Iga are pretty cool designed items, but they're stupidly strong. If you lowered the damage of their procs by a lot (so they do, 50/100 damage per proc instead of 300 or so), then that'd be fun. I'm not so sure about the status immunities, it might be too strong when paired with other items. I'd need more data, aka field testing to see that.