The more I think about the warlock should be in a later expansion. That way the level of detail the class needs can be addressed fully. Also I think it's flavor is too outlandish for the base game (my opinion).
Well, here's the thing... thus far it sounds as though there are only going to be four classes that do not have "outlandish" flavor-- the Fighter, Rogue, Cleric, and Wizard. Those four will be the more 'standard' classes. The same way the human, elf, dwarf, and halfling will be the four more 'standard' races. They will all have a fairly standard level of specificity and detail... enough to make them interesting, but not so much that they can't be plugged into almost any setting, or so weird that they seem more... "advanced"(?)... if that's the way you'd put it.
Every OTHER class or race has been indicated to be further "out there" than those four base ones (even those classes that traditionally have been more 'standard' in previous editions). And by "further out there", they mean that they will have a greater level of specificity in terms of fluff (the Rangers having a code of conduct and an organizational structure... the Warlocks having Patrons and Pacts that are beyond something like "Devil!" or "Archfey!"... the Tieflings being more setting specific, the Halforks or Drow not being nearly as populous in any setting, etc. etc.)
So the question then becomes... what appears in that very first Player's Handbook? JUST the four standard classes and races... or those four PLUS a number of the more esoteric classes and races (of which the Warlock applies)?
For my money... I don't see how you could possibly NOT include more of the esoteric ones in the first Player's Handbook. Because those will all seem rather cool, and fun to play and look through. Plus, you need SOMETHING to put in the PH... and I don't expect there to be magic items or such a huge list of spells in comparison to previous books.
But for those people who want just the "basic version" of D&D, with JUST the standard four races and classes? The answer is quite simple-- BEGINNER'S BOXED SET. Levels 1 to 5... only the F/R/C/W and H/E/D/Hlg. THAT'S where keeping the core standard and more simple makes the most sense.