I dont think I have ever touched a "challenge mission" in a single game that has had them so its a little lost on me.
I havent played a single Souls game all the way through but I will definitely be getting Bloodborne as it looks like a new IP and seems to be a little more orientated to my tastes. However it does have little appeal to me, I havent heard of any story elements, I dont think the character even has a name which is fine, I get the point of it. But its different it seems to me its had a comparitively short dev time compared to The Order and I expect it to have various areas and enemies and heavily rely on gameplay and tactics.
OK but its story will be next to non-existant. After all it looks to be an action game with a short dev time and what would seem to be a B-team of From Software. In my opinion and from what Ive seen Bloodborne will heavily rely on repetative "kill-X, Kill-Y but with different sets of rules, such as enemy X had a shield and enemy Y has flame breath" I dont see how thats of any more value than a deep engaging story.