The question you should ask is: "What role should these items fill?"
Do you want them to be niche-items good on just a few champions, or "standard" items that can be built by many champions? Entropy could be remade into an item for AD carries that want some slow. In that case, you might want to consider removing the true damage over time, since it doesn't really mesh well with most AD carries' kit. On the other hand, Entropy could be made as a bruiser item for bruisers than don't quite want Frozen Mallet but do want some slow, hp and damage. In that case, having true damage might not be so bad.
KItae's Bloodrazor could be made as a niche on-hit item that's good for a few champions, such as Warwick. Or it could be made as an alternative for AD carries that want to deal with people stacking hp. The latter, however, might be hard to balance. Due to the way crit works, passive on-hit effects usually won't benefit AD carries all that much. Furthermore, the passive being magic damage won't help most AD carries, since they tend to get armor pen. You could consider giving it physical damage instead of magic damage, or perhaps even true damage. True damage might be hard to balance, though. You could add an mechanic similiar to Vayne's Silver Bolts, but that might make Vayne feel less special, and the item might be problematic when Vayne buys it.
Due to all these factors, I think Kitae's should probably be used as a niche-item used for champions that want an on-hit effect that deals magic damage. Perhaps you could reduce or remove the AD and instead give it some magic pen? This, however, might make it a bit too similar to Malady. I'm not really sure what to do about it.