Why is it so bad that some creatures/NPCs get Stat blocks? 
I don't remember where i first heard it but i've often heard the phrase "If you stat it, they will (try to) kill it", but it's one i've seen often enough that it partly rings true. I've seen plenty of forms where "Try to kill Tiamat/Tarrasque/Another high CR creature as short as possible" is the subject and people come up with ideas like a party of divination wizards mixed with paladins/rouges, and number crunching. It seems like a very fun thought experiment to try and deal the most dammage possible to such a challange, but the odds of ever running into such a being are very low; they are meant to be a High Level Challange for PCs and players who have proven to work well together and effeciently. To get to such a level of experiance both in and out of game requires months if not years of time in a single campaign that hopefully doesn't get stale. So for the majority of players, they may never see a Tarrasque or Emperyion or Tiamat unless they play a published campaign such as Tyranny of Dragons.
Now come fall we are begining the Rage of Demons storyline where the Underdark and various demonic/devilish entities will be coming up from Demogorgon to Drizz't (who gets included because while i've never read any of Salvadore's books I've seen enough complaining that he is a demon to some people 
ha ha).
This is where the initial topic of the thread comes into play, where entities who have the ability to be the Patron of a PCs Warlock pact will be seen and possibly fought against, which requires them to be "statted". Personally i feel interested in seeing how "powerful" some of these creatures will be but does that take away from the fun of gameplay, if all people care about is how high are their numbers rather than becoming immersed in a theatre of mind or even miniature style of gameplay?
I will be very interested if WotC decides to create a Player's Compendium much like they did for Princes of the Apocalypse and what will be included for Warlock characters, whether it be simply Fiend Pacts that get expanded on or will we see Archfey Queens/Hags and Great Old Ones trapped underground, what mechanics that will add and how will it impact the flavor of an already flavorful class, perhaps even answering questions like "Can a Warlock lose his abilities like a Paladin?" or "Can a Warlock's Patron be slain?".
In 4e there were interesting concepts for killing your patron and supplanting him or her. Several epic destinies, in fact.
For Warlocks I really hope they do something for pact blade warlock like give them a invocation to duel wield pact blades. Or allow them to wear leather armor while using the shadow armor spell for the invocation. Other than that it would awesome if your feind pact warlock gets effected in the expeditions or campaign guide if he has a pact with Graz'zt, Demogorgon, or Orcus!