Nothing wrong with a mystery box, as long as 1) there actually ARE answers, and 2) the DM/author/screenwriter doesn't keep gratuitously smacking the audience (e.g. PCs) upside the head with it. Sadly those conditions are too much to ask for some.Maybe don't decide what's going on with the orcs, but try to make it a mystery (i.e. make them a /sigh mystery box). There are tough organized raiders who it is said destroyed the dwarves and elves in ages past, and who are known to unexpectedly form invasion armies, but who come from an unknown homeland, and have unclear motivations. You can have the evil marauding humanoids, but throw in unexpected signs of greater complexity when the PCs interact with them. Then you listen to the players speculate about them and steal liberally when/if they have good ideas.
Eh, maybe that's a difficult lift given how done to death the evil orc horde archetype is.