First thing to consider is what role the pet would play. Everything but healer has been done already, and I don't want to even think about a Warlock with a pet healer. The Felguard is essentially a direct upgrade to the Voidwalker, so something like that is more likely. Personally, I think a Shivarra would fit the next demon well, and act as a direct upgrade to the Succubus.
Second thing to consider is whether it would make sense from a lore point of view. As fun as it would be to summon a pet Pit Lord whenever you want, that seems unlikely. Same with the Nathrezim or the Eredar. Shivan or something we haven't seen yet seems the most likely choice.
Its possible they could go a different route entirely, and despite my enjoyment of the shivan idea, this is my personal favorite theory. Instead of giving a brand new baseline demon to Warlocks, I'm in favor of giving them a more unique spell that would fall into this area. As a sign of their mastery of demonology, the Warlock becomes capable of summoning a great number of lesser demons, rather than simply one bigger one at a time. What I mean by that is something similar to the Moonkin's treants, or a Snake Trap. Place a summoning portal, and a train of low health imps flow out to rain firebolts at your enemy for awhile. I'll leave the numbers game to the devs. Unless they want to pay me to do it, of course.*
Why is this my favorite theory? For one, its quirky. Opening a nether portal in the middle of combat is pretty sweet. Its up there with fel fire for the 'locks on the hierarchy of awesome things. Two, its a fun and effective toy to give the Demonologists, which leaves room to rebalance the 'useless' pets. They can give the voidwalker, infernal, and the doomguard the love and polish they so desperately need without worrying about the new demon encroaching on their territory. Third, opening a nether portal in the middle of combat is pretty sweet. Did I say that already?
That's just my two cents on it, really. Most everyone has ideas they'd like to see or opinions on what their class needs or what color sparkles their class should have. Trying to guess what's around the corner is never dull.
*Psssst, mail me. I work for sandwiches.